Panasonic Lumix DMC FZ50 Digital Camera

Panasonic Lumix DMC FZ50 Digital Camera
Nikon TC-E15ED and TC-E17ED Teleconverters
Resolution Monochrome and Colour
and Vignetting Assessments
Dr James C Brown CEng FIMechE
Contents
1.
Introduction
Resolution tests – Olympus C-3030Z
2.
Image Quality and Resolution
The limit of resolution
Graphical simulation
3.
Resolution Tests - Panasonic FZ20
4.
Test Chart Design
5.
FZ50 Resolution tests
Resolution Test Charts
Test Method
Resolution Assessment
Analysis of the test data
6.
FZ50 Resolution Test Results Colour
The effect of colour
Comparisons for black and six colours
7.
Discussion and Conclusions
Black and white images
Coloured images
8.
TC-E15ED Vignetting
Vignetting Test Results – Images
Vignetting Test Results – Plots
EZ Zoom
9.
Resolution Tests with Nikon Teleconverters
Resolution at 420 mm with and without TCs
Comparisons for black and six colours
10.
Resolution with Single and Stacked Teleconverters
Test Method
Resolution test results - images
Resolution at 420 mm with and without TCs
Comparisons for black and six colours
11.
Discussion - Teleconverters
Individual TCs
Stacked TCs
Copyright © 2010 James C Brown
1
1. Introduction
This project started out initially as an investigation of the resolution of a 3 MP digital camera
for comparison with that of my 35 mm film SLR, the results of which led to the additional
investigations described in this document.
These included an attempt to find an explanation for the measured resolution of the digital
images being much lower than I expected, the effect of the edge of an image overlapping
adjacent pixels and the influence of the Bayer matrix on the resolution of coloured images.
Having used 35 mm film for over 40 years, when I was given an Olympus Camedia C-3030Z
in 2001, I was curious to see how its resolution would compare with that of my SLR.
From the tests I’d done previously to assess the resolution of my Minolta SLR with its 50 mm
standard lens and Sigma zoom lenses, I knew that their central resolutions ranged from 40
to 120 LPM, line pairs per mm.
From these figures I calculated that, for the minimum resolution of 40 LPM, the maximum
number of lines which could be resolved in the 24 mm height of a 35 mm film image would
be 1920 (24 x 80), considerably higher than the 1536 pixels in the height of the (2048 x
1536) 3 MP digital image of the C-3030Z.
Assuming that at the limit of resolution, a digital camera could resolve one line per pixel, the
1536 pixel image height of the sensor should correspond to 768 line pairs which, for the 24
mm height of a 35 mm image, would correspond to a resolution of only 32 LPM (768/24),
about 75% of the minimum resolution of my film SLR.
Resolution tests – Olympus C-3030Z
While I was carrying out tests on my Minolta film SLR in the late 1980’s, I was given a copy
of the Fujifilm Resolution Test Chart shown below. As will be seen from the image of that
chart it contains three sets of resolution test grids. These grids have scales of 1.8, 2.0, 2.2,
2.5, 2.8, 3.2, 3.6, 4.0, 4.5, 5.0, 5.6, 6.3, 7.1, 8.0, 9.0 & 10.0 LPM.
Fujifilm Resolution Test Chart
2
As a check on my calculations I used my Olympus C-3030Z to take shots of the above test
chart using a scale factor of 10. In that way the 35 mm equivalent resolution could be
obtained simply by multiplying the resolution value of the highest distinguishable LPM
pattern on the image by 10. That is illustrated in the following crop taken from a digital image
of the test chart.
Crop from the digital image of Fujifilm Test Chart shot with the Olympus C-3030Z
I was surprised to find that, as will be seen from the above crop, the resolution obtained from
these tests was only about 22 LPM, 2.2 LPM x10, much lower than my calculated value of
32 LPM and just over half the lowest value for my film SLR.
As will be seen from the following images the appearance of the digital image was also very
different from that of the image on 35 mm film. In the crop taken from the digital image, the
lines consist of strips of black and various shades of grey, whereas the edges of the lines in
the image shot on film are clearly defined as either black or grey. The high magnification
shot of the 35 mm film was taken using my FZ50 with both Raynox 150 and 250 close-up
lenses attached. This can be clearly seen by comparing the following crop from a digital
image taken with the C-3030Z with the crop taken from the digital image of the 100 ISO
Fujichrome slide film beneath it, which shows a resolution of at least 45 LPM.
Top part of the crop from the C-3030Z digital image of the Fujifilm Test Chart
3
Magnified Crop from a digital image of a 100 ISO Fujichrome slide of the Fujifilm Test Chart
As described in “Image Quality and Resolution”, by using a graphical method to simulate
the interaction between the image of black and white grid pattern and a row of light sensitive
pixels, I found what I consider to be a plausible explanation for the unexpectedly low
resolution of the digital images shot with the C-3030Z. That simulation led me to conclude
that the low resolution I’d observed was due to the pixels of the sensor being partially
overlapped by the edges of the lines in the image.
The results of the graphical simulation also indicated that for a typical digital sensor, in order
to clearly resolve one line pair three pixels are required, i.e. 1.5 pixels per line width. That
conclusion relates closely to the ratio of the calculated resolution value of 32 LPM to the 22
LPM resolution assessed from the image of the test chart taken with the Olympus C-3030Z.
4
2. Image Quality and Resolution
While there are many factors which influence photographic image quality, for many types of
photography, “resolution” the ability to resolve fine detail in an image is critical. The aim of
this project was primarily to assess the limit of resolution for my own cameras and lenses.
The limit of resolution
As discussed in http://webvision.med.utah.edu/KallSpatial.html, the generally accepted limit
of resolution for the human eye is 1 arc minute. At the limit of visual acuity the thickness of
the strokes and spaces in the letter E of a visual acuity test chart correspond to an angle of 1
arc minute. Therefore at the limit of resolution the letter E has a total angular height of 5 arc
minutes.
In Section 5 of the above reference “Dimension of the retinal mosaic.” the maximum
resolution is discussed in relation to the spacing of the retinal cones in the human eye and
includes the following statement:
“Helmholtz proposed that a grating would be resolved if there is a row of unstimulated cones
in between rows of stimulated cones. This is considered the Yes-No-Yes response of the
cone receptors.
For example, if two lines are to be resolved, a detector array needs to be fine enough to
detect a gap in between the two lines (figure 13). From figure 13, it can be seen that
detectors A and B will not be able to resolve the two lines. However, with a fine detector
array of detectors C, D, E and F, the two lines would be resolved.”
In the case of the human eye, if the lines are not in exact alignment with the cone receptors
on the retina, interaction between the eye and the brain will result in the eye moving so as to
bring them into alignment. Consequently the theoretical limit of resolution for a healthy
human eye will be one line per receptor.
Unlike the human eye, digital cameras don’t have the ability to track such an image so as to
bring the lines into exact alignment with the rows of pixels. Consequently the edges of the
lines of the image may overlap the adjacent pixels to some extent and so result in these
pixels detecting shades of grey which depend on the degree of overlap as illustrated below.
5
Graphical simulation
The following images illustrate graphically the responses of a detector with a pixel width L
when grids of various widths W have their positions changed in steps of 0.25 L relative to the
positions of the pixels in the sensor.
Figure 1 Line width W = 0.5 Pixel length
The above image shows that for a grid with a line width of 0.5 L none of the grid lines will be
detected.
Figure 2 Line width W = 0.75 Pixel length
The above image shows that for a grid with a line width of 0.75 L only every second line will
be detected.
6
Figure 3 Line width W = 1.0 Pixel length
The above image shows that for a grid with a line width of 1.0 L detection of the lines
depends on the degree of overlap. As a result the grid lines may or may not be detected and
if detected both their intensity and position will vary with the degree of overlap.
Figure 4 Line width W = 1.25 Pixel length
In the above image for a grid with a line width of 1.25 L, the black lines are detected
alternately as either black or dark grey and separated by either a single white space or by
two spaces of different shades of grey. The number of grid lines detected can be regarded
correct only if the light grey and or mid grey pixels are regarded as spaces. However in
either case the position, width and intensity of the lines detected depends on the degree of
overlap and the pattern is a poor representation of the original grid.
7
Figure 5 Line width W = 1.5 Pixel length
The above image shows that for a grid with a line width of 1.5 L, the number of grid lines
detected is correct. However the position, width and intensity of the lines detected depends
on the degree of overlap and none of the patterns is a true representation of the original grid.
Figure 6 Line width W = 2.0 Pixel length
The above image shows that for a grid with a line width of 2.0 L, the number of grid lines
detected is again correct with the detected width, position and intensity depending on the
degree of overlap. It is however a somewhat more accurate representation of the original
grid pattern.
8
Figure 7 Line width W = 2.5 Pixel length
The above image shows that for a grid with a line width of 2.5 L, the number of grid lines
detected is again correct with the detected width, position and intensity depending on the
degree of overlap. It is also a more accurate representation of the original grid pattern.
Figure 8 Line width W = 3.0 Pixel length
The above image shows that for a grid with a line width of 3.0 L, the number of grid lines
detected is again correct with the detected width, position and intensity depending on the
degree of overlap. It does however represent the original grid pattern much more closely.
As illustrated by the comments beneath them, examination of these images led me to
conclude that, due to the effect of the edges of the lines of the grid partially overlapping
adjacent pixels, the resolution of a line pair, i.e. one black line and one white line, requires
three pixels. In that case the limit of resolution for the sensor in a digital camera would be 1.5
pixels per line width, which is just 2/3 (67%) of the resolution which would be predicted from
the “Yes-No-Yes” response proposed by Helmholtz.
9
3. Resolution Tests - Panasonic FZ20
In October 2004, having read the DPReview preview, I bought a Panasonic FZ20. Keen to
assess its resolution, I took several shots of the Fujifilm test chart which was described
above in relation to the tests on the C-3030Z.
As will be seen from the following crops from these images the resolution was found to be
about 28 LPM which, for the 24 mm height of a 35 mm image, corresponds to a vertical
resolution of 1344 LPH, lines per picture height. Based on the requirement of three pixels
per line pair as suggested by the graphical simulation, the 1920 pixel height of the FZ20
sensor would correspond to 640 line pairs. That corresponds to a resolution of 1280 LPH
which is very similar to the value assessed from the Fujifilm test chart.
Crop taken from a digital image of the Fujifilm Test Chart shot using the FZ20
Magnified crop from the digital image of Fujifilm Test Chart which shows the resolution of the
FZ20 to be about 28 LPM
10
When it was released by DPReview in November 2004, Simon Joinson’s review of the FZ20
stated the vertical resolution of the FZ20 to be 1300 LPH, which is also very similar to the
above values. See: http://www.dpreview.com/
That review of the FZ20 includes crops taken from the image of the test chart which are
used to assess the horizontal and vertical resolution. Examination of these crops at high
magnification reveals more clearly the effect of the edges of the tapered lines partially
overlapping adjacent pixels in the sensor.
These overlaps cause the width of both the lines and spaces to vary by one or two pixels so
that the image is not an accurate representation of the test pattern in which the lines and
spaces are of equal width. In that respect there is a clear similarity between the images in
the DPReview test chart, the above images and those in the graphical simulation discussed
in “Image Quality and Resolution”.
In October 2007, I decided to do some further tests on the vertical and lateral resolution of
my FZ20 using the test charts which I’d used to test the lenses of my Minolta film SLR and
the Fujifilm resolution test chart simultaneously.
Fujifilm Resolution Test Chart and Test Grids shot at 12X Zoom from a distance of 6.35
Metres with the FZ20
Crop taken from part of the Test Grids shown in the above shot taken with the FZ20
11
When I examined these images in detail I was surprised to see coloured bands along the
edges of the black lines. As shown below, these can be even more clearly seen in the
magnified crop taken from the image.
Crop taken from a magnified crop for the above image of the Test Grids
The notes on the construction of sensors in the “Learn” section of the DPReview website
indicate that sensors based on the Bayer matrix, use a pattern of one red, one blue and two
green filters arranged in a square. The colour and intensity values assigned to each
individual pixel are obtained by combining the intensity and colour of the output for that pixel
with those from several of the pixels surrounding it. See http://www.dpreview.com/
Arrangement of the three primary colours in a Bayer matrix and the possible relative
positions of the red, green and blue pixels
The top part of the above figure illustrates for part of a sensor the relative positions of the
red, green and blue filters of the Bayer matrix. The lower part of the figure shows for red,
green and blue filters the colours of the filters for the eight pixels which surround it. Note that
in the case of the green filter, there are two alternatives for the positions of the red and blue
filters.
12
In principle, if the colour of the light falling on a pixel matches the colour of its filter the
situation is fairly straightforward. However if the colour of the filter is such that it blocks some
or all of the incoming light, for example any combination of blue and green light falling on a
pixel with a red filter, the situation is much more complicated.
In general entirely arbitrary amounts of red, green and blue light will fall on any individual
pixel which may have a red, green or blue filter. The demozaicing software used to
determine the colour and intensity values to be assigned to that pixel must derive the
required information from the light received by that pixel through its red, green or blue filter
and from the light falling on the pixels which surround it through their respective filters.
That information led me to suspect that the colour fringes at the edges of the grid lines could
be due to the combined effect of the pixels being partially overlapped by the image and to
the way in which the colour and intensity data assigned to each individual pixel is obtained
and to conclude that as a result the resolution of digital cameras whose sensors are based
on the Bayer Matrix is likely to vary significantly with colour. In view of that, I decided to try to
make a test chart which would allow me to investigate the limit of resolution of my cameras
and the extent to which it varied with colour.
13
4. Test Chart Design
In order to investigate the effect of colour and improve the measurement accuracy I used
Photoshop Elements with a resolution of 200 pixels per cm to draw and print a test chart
consisting of groups of five equispaced colour strips in the form of a grid.
The lines in the grid have a height of 10 mm and thicknesses ranging from 0.2 mm to 1.0
mm in steps of 0.05 mm, then in steps of 0.25 mm to 2.5 mm. The groups of five coloured
lines are separated by groups of five black lines and are repeated in a sequence of red,
green, blue, magenta cyan and yellow.
Colour Test Grid and a crop from a shot taken with my Panasonic FZ20
In the above image, the top part of is a copy of the bottom four lines of the test chart and the
bottom part is an equivalent crop from an image of the chart which was taken with my FZ20
mounted on a tripod. The shot was taken from a distance of 5 metres at the maximum 35
mm equivalent focal length of 432 mm with an aperture of F2.8 using Auto White Balance at
ISO 80.
As will be seen from the bottom part of the image, none of the 0.20 mm grid lines in the
bottom row have been resolved. In the second row the 0.25 mm black and red lines and
some of the magenta and blue lines have been resolved. In the third row the 0.30 mm black,
red, blue and magenta lines have been clearly resolved and some of the green lines have
been resolved. In the top 0.35 mm row the black, red, green, blue and magenta lines have
been clearly resolved and most of the cyan and yellow lines have been resolved. These
results confirmed that the resolution of my FZ20 varied significantly with colour.
The demand for high resolution in photographic images arises mainly from the desire to be
able to see the fine details in a subject, to make large prints from these images or to be able
to severely crop images of small remote subjects such as wildlife.
Bearing in mind both those requirements and the issues relating to visual acuity testing as
discussed in “Image Quality and Resolution”, I decided to design a test chart based on a
letter E with the standard proportions specified for visual acuity tests. I concluded that a
suitably designed chart should meet the joint requirements of being related to the resolution
of the human eye and those required to evaluate the performance of the lens plus sensor
combination of a digital camera with both colour and black and white images.
14
As discussed in http://webvision.med.utah.edu/KallSpatial.html, the generally accepted limit
of resolution for the human eye is 1 arc minute. For the very small angles involved in relation
to the limit of resolution I find it much more convenient to work in radian measure.
In radian measure, the size of the angle contained between the ends of an arc of a circle is
equal to the length of the arc divided by its radius. For circle of radius R the circumference is
2xπxR thus the contained angle of 360 degrees is (2xπxR)/R = 2xπ radians and the 180
degree contained angle of a semi-circle = π radians.
Since π radians = 180 degrees, 1 degree = π/180 radian = 0.0175 radian and 1 arc minute
is π/(180x60) = 0.00029 radian or 0.29 milliradian. For small angles of a few degrees the
difference between the length of the arc and the length of the chord joining the ends of the
arc is negligible. Hence the angle subtended by the thickness of the lines in a letter E is
equal to the thickness of the lines divided by the distance from the letter E to the eye or to
the optical centre of the camera lens.
The 1 arc minute limit of resolution which is equal to 0.29 mR (milliradian) is therefore
equivalent to a line thickness of 0.29 mm at a distance of 1 metre or 2.9 mm at a distance of
10 metres and so on. Thus the use of radian measure makes the calculation of resolution
data much simpler and easier to follow
The same principle can be applied to the image side of the lens. Thus for a lens focussed at
infinity, the angle subtended by a pixel on the sensor is equal to the height or width of a pixel
divided by it distance from the optical centre of the lens, which when focussed on infinity is
its focal length. That is illustrated in the following table for the FZ20 and the FZ50 at the focal
length lengths corresponding to minimum and maximum zoom. Note that for subjects closer
than infinity the distance from the optical centre of the lens to the image will be slightly
greater than the focal length.
Comparison of linear and angular resolutions for FZ20 and FZ50
Since, with a few exceptions, light travels in straight lines, there is a simple linear
relationship between the size of a subject and the size of its image on the surface of the
sensor and vice versa. It is therefore possible to estimate for a subject at any distance from
the camera what the size of its image on the sensor will be in terms of the number of pixels it
covers. Similarly it is possible to estimate the size of a pixel projected to the position of the
subject.
The design of the various test charts I used to assess the resolution of my cameras was
based on that principle. In my first design based on the letter E the line thickness ranges
from 0.2 mm to 1.0 mm in steps of 0.1 mm as shown in the following enlarged image.
15
Colour test chart based on the letter E used in visual acuity tests.
In the above image, with the resolution set to a value of 100 pixels per cm, I used the
‘Rectangular Marque” and “Paint Bucket” Tools of Photoshop Elements to create the
individual Es of each size and colour and print the test chart.
From the test results obtained with the above chart I concluded that the 0.1 mm step
increase in the thickness of the lines in the letters was still too coarse. That led me to make a
replacement chart in which the thickness of the lines in the Es ranged from 0.25 mm to 1.25
mm in steps of 0.05 mm. To allow me to relate the thicknesses of the lines in the chart to the
size of the image in pixels I added a 100 mm vertical black line to the chart.
When further tests led me to conclude that the 0.05 mm step increase was still not quite fine
enough I decided to try to relate the resolution and the line thickness to the height of a pixel
and used Excel to create the following plot for the FZ50, for which the resolution in LPH =
2736/ line thickness in pixels.
Plot of resolution in LPH vs. line thickness in pixels
16
My intention was to design a new version of the chart in which the thickness of the Es in
each successive row would correspond to a 50 LPH difference in resolution. Unfortunately
from the values of the line thickness data in the above plot it was clear that with my limited
resources it would not be possible for me to print such a chart with the precision required. In
view of that I decided that the next best option was to use Photoshop Elements with a
resolution setting of 400 pixels per cm to create a new version of the chart in which both the
thickness of the lines in the smallest Es and the row by row increase in their thickness would
be related to the projected height of a pixel.
The results obtained from my examination of the images of the new chart suggested that my
assessments of the limit of resolution were somewhat variable. In order to demonstrate that
it was due to the effect of the edges of the images of the Es overlapping adjacent pixels I
devised a new black and white version of the chart. The detailed design of both these charts
is described and discussed in “FZ50 Resolution Tests”.
17
5. FZ50 Resolution Tests
Resolution Test Charts
As described in “Test Chart Design” I experimented with several alternatives before settling
for the design of the final colour test chart. The design is based on the same principle as the
visual acuity charts used by optometrists for testing the human eye. However to allow
assessment of the effect of colour it is constructed from both black and coloured letters. The
chart was constructed in stages from individual rows of Es of different sizes which had been
created using Photoshop Elements at a resolution of 400 pixels per cm. Each row consists of
the letter E printed in red, green, blue, black, magenta, cyan and yellow.
In the final version illustrated below, a line thickness 0.25 mm was chosen for the smallest of
these letters as being the most convenient value which was greater than the 0.2 mm
minimum at which my printer could reliably print lines and spaces of sufficiently equal width.
The thickness of the lines in each successive row of Es was increased by 0.025 mm, i.e.10%
of the thickness of the lines in the first row.
Resolution Test Chart – Coloured Es - Line thickness 1.0 to 3.5 pixels
The 0.25 mm thickness of the lines was chosen also as a convenient value with which to
match the projected height of a single pixel when the distance from the test chart to the
camera had been adjusted so that, for a chosen focal length, the 100 mm scale line on the
chart had a length of 400 pixels in the image.
In a manner similar to that discussed under Graphical simulation in Image Quality and
Resolution, the test chart below was designed to confirm the effect on the resolution of the
edges of the lines of the image partially overlapping adjacent pixels.
Photoshop Elements was used to construct the chart in stages from rows of Es of different
sizes which were created separately. The first row consisting of three groups of 11 Es was
created by displacing each successive letter E in the group upwards by 0.1 of a pixel, i.e. by
one tenth of the line thickness of the smallest E, so that the last letter in each group of 11
was 10 pixels higher than the first.
18
Resolution Test Chart - Stepped Es - Line thickness 1.0 to 3.5 pixels
To maintain the same gradient in all the subsequent rows, the number of pixels by which last
E in each group of 11 was raised was increased by 1, i.e. 11 pixels for the 1.1 pixel row, 12
pixels for the 1.2 pixel row and so on. Unfortunately without increasing the already very high
resolution of the chart by a further factor of 10, that could only be achieved by raising several
of the letters in each group of 11 by an extra increment. As a result the proportion of the line
thickness by which each letter is raised varies slightly from the ideal of 0.1 of the line
thickness in all but the 1.0 and 2.0 pixel rows
To represent the projected size of the 3648 x 2736 pixel sensor of the FZ50, for a 0.25 mm
projected pixel height, a 912 mm x 684 mm rectangle was drawn a board which had been
painted matt white. Copies of the test chart were attached at the centre, four corners and
centres of the edges of the rectangle.
Resolution test board
19
Test Method
Resolution test arrangement showing relative positions camera and test chart
The above image shows the arrangement used for taking the test shots. The test chart was
clamped to a vertical surface at a convenient height and adjusted for level. For each focal
length tested the FZ50 was attached to a tripod and its position and attitude adjusted so that
the camera was level, the optical axis of the lens was perpendicular to the test chart, level
with and pointing at its centre. Its distance from the chart was also adjusted carefully until the
length of the 100 mm scale in the image was 400 pixels, within a few percent.
After turning off the OIS, setting the white balance manually and adjusting the exposure, I
used a remote shutter release to take shots at -0.33EV, 0.0 EV and +0.33 EV, for each half
stop interval from maximum to minimum aperture.
Resolution Assessment
After transferring all the images for a chosen focal length to my PC, I took a 300 x 550 pixel
crop from the centre of each image and transferred these to folders for subsequent
assessment. The limit of resolution for each colour was assessed by examining the crops
from each set of images shot at the three different EV values and looking down each column
of letters in turn to select the minimum line thickness at which the three horizontal strokes of
the E of each colour could still be recognised.
This is illustrated below in a typical set of 300 x 550 pixel crops and in the magnified sections
cropped from these to show a typical set of assessed resolution limits in which the values
selected are underlined in red. As will be seen from these images assessing the resolution
for each requires a subjective judgement which in some cases can be rather difficult. The
following images are from shots of the test chart which were taken from a distance of 3.50
metres with a focal length of 135mm at F/4.
20
300 x 550 pixel crops from shots which were taken from a distance of 3.50 metres with a
focal length of 135mm at F/4
Magnified sections of the crops from the 135 mm F/4 shots marked in red to illustrate a
typical set of assessed resolution limits
21
Part of the stepped Es test chart compared with the corresponding part of an FZ50 image
the test chart
The image above is a crop from a PC screen capture in which Photoshop Elements was
used to allow comparison of an image of the black and white test chart of stepped Es with
the corresponding part of the PSE image which was used to print the test chart. The area
selected includes the rows of Es with a line thickness corresponding to 1.2 to 1.7 pixels in
the image.
As can be seen from the crop of the image on the right, none of the Es in the 1.2 or 1.3 pixel
rows are recognisable. Only some of the letters in the 1.4 pixel row can be recognised as Es.
While all of the letters in the 1.5, 1.6 and 1.7 pixels rows can be recognised as Es, their
clarity varies with their positions along the length of the row. That result appears to confirm
that the limit of resolution is significantly affected by the effect of the edges of an image
overlapping adjacent pixels. It also provides a plausible explanation for the amount of scatter
in the assessments of the limit of resolution in the coloured images.
Analysis of the test data
An Excel spreadsheet was used to set up tables in which to enter the relevant data,
including the image numbers, for each set of images. The resolution value for each of the
colours, as assessed from the individual crops for each aperture at each focal length, was
entered in these tables, an example of which is shown below.
Table of resolutions in pixels per line
22
As illustrated in the table below by dividing 2736, the number of pixels in the height of an
FZ50 image, by the line thickness assessed as the limit of resolution, the resulting data was
converted to an equivalent resolution in LPH and used to prepare plots of resolution vs.
aperture.
Table of resolutions in lines per picture height, LPH
By combining the results from the various data sets it was possible to plot the results in
several different ways. These include the variation with colour and aperture for each of the
three focal lengths individually and for all three focal lengths, a side by side comparison of
the variation of resolution with aperture for black and each of the six colours. These plots are
shown in “FZ50 Resolution Test Results”.
23
6. FZ50 Resolution Test Results
The effect of colour
To illustrate the effect of colour for each of the individual focal lengths 35, 135 and 420 mm,
the following plots show for black and each of the six colours, red, green, blue, magenta,
cyan and yellow, the variation of resolution with aperture.
As will be seen from all three of the above plots, there is a significant variation of resolution
with colour which is similar for all three of the focal lengths chosen for these tests.
24
It is also clear that for all colours the resolution decreases in a similar way with increasing F
Number. That is probably due to the effect of diffraction which increases as the aperture is
reduced. However while the resolution at maximum aperture is slightly lower than when
stopped down by half a stop, there is no evidence of a significant optimum aperture, ‘sweet
spot’, in any of the plots.
Though the difference is not great, it is clear that the resolution is highest for the red and
black Es, with magenta and blue slightly lower, followed by green then by yellow and cyan
which show the poorest resolution. Part of that variation may however be due to differences
in the relative intensities of the individual colours on the printed chart, which would of course
depend on the accuracy with which my Canon printer printed the colours.
Comparisons for black and six colours
The following plots show for black and the six colours, the variation of resolution with
aperture for focal lengths of 35, 135 and 420 mm. In these plots, for all three focal lengths,
the shape of the variation with aperture is fairly similar for all colours. However there are
significant differences in the maximum resolution for each colour.
Due to the effect of the edges of the images of the Es overlapping adjacent pixels assessing
the limit of resolution for digital images is a sometimes a difficult subjective judgement.
Consequently there are some fairly obvious irregularities in these plots. In addition the
values derived from all of these tests will be directly affected by the degree to which the
edges of the lines used to construct the individual letters overlap the adjacent pixels.
Taking some account of these I concluded that, as a reasonable approximation the black and
red and magenta plots show the highest resolution at a maximum of about 1950 LPH,
followed by blue and green at around 1800 LPH then by yellow at 1500 to 1700 LPH and
cyan at 1400 to 1600
25
26
27
7. Discussion and Conclusions
Black and white images
The maximum vertical resolution value of 1950 LPH which I assessed for the black Es in the
images from my FZ50 tests is fairly close the absolute vertical resolution value of 1800 LPH
given by Simon Joinson in his review of the FZ50 on the DPReview website.
http://www.dpreview.com/
When, several months after I’d completed my tests with my resolution test chart of coloured
Es and assessed the images I was given a copy of an ISO12233 resolution test chart in
Adobe Acrobat PDF format, I decided to try to reproduce the results of the FZ50 resolution
test published by DPReview.
By importing the ISO12233 PDF file into Photoshop Elements I was able to select the
vertical resolution test pattern and print a copy at a suitable size. I then shot several images
of the chart from the distance required to give the correct scale. The following image shows
a screen capture from PSE of the vertical resolution test pattern cropped from one of these
test shots. To show the detail more clearly and make the image fit the display of a PC it has
been divided into two sections.
As will be seen from the above image, due to the effect of the edges of the image
overlapping adjacent pixels, the lines and spaces become progressively less distinct as the
line thickness reduces. This effect can be seen more clearly in the following image in which
the upper part is a copy of the high resolution end of the ISO 12233 vertical resolution test
pattern with its scale reduced to match that of the FZ50 image. The position of the 1800 LPH
limit of resolution published by DPReview has been marked in red in both.
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High resolution end of ISO 12233 resolution test chart and enlarged crop from equivalent
FZ50 image with1800 LPH resolution position marked in red
Under ‘Resolution’ in the ‘Learn’ section of DPReview, the limit of resolution is defined as the
point up to which the black and white lines can be distinguished from one another. On that
basis, from the above image of the ISO12233 test chart I would assess the limit of resolution
of my FZ50 to be at least 1850 LPH. Due to the nature of the image that is however a very
subjective judgement.
Also as discussed in relation to the DPReview resolution tests of the FZ20, as the width of
the lines and spaces is progressively reduced, the effect of the edges of the image partially
overlapping adjacent pixels causes the width of both the lines and spaces in the image to
alternate between one and two pixels with the result that as it approaches the limit of
resolution the image becomes an increasingly less accurate representation of the test
pattern in which the lines and spaces are of equal width over their full length. That effect can
be clearly seen by starting at the right hand end of the above image and observing the
appearance of the individual lines as you follow them towards the left. By so doing you will
see that the appearance of the lines and spaces varies considerably along their length.
At some positions you will notice that some of the lines are in the form of a single row of
black or very dark grey pixels separated by spaces which consist of two pixels of different
shades of grey while others at the same position consist of two pixels which can be equally
dark or of one dark and one lighter giving the illusion of a three dimensional 'Vee'. As these
two pixel width lines are mostly separated by spaces consisting of a single row of pixels they
clearly do not accurately represent the original image in which the widths of the lines and
spaces are equal.
That observation leads to the obvious question of how the irregular edges of lines and
spaces which vary by one or two pixels affect the visual appearance of more typical
photographic images. As discussed in relation to the Olympus C3030Z resolution tests, the
difference in the appearances of the grid lines in the digital and film images of the Fujifilm
chart led me to conclude that the quality of the detail in a digital image would be lower than
would result from a film image with an equivalent traditional LPM resolution and would result
in a loss of definition in fine details such as a cat’s whiskers or the eyelashes in a portrait.
In the enlarged section of the image of the ISO 12233 resolution test chart, in addition to the
appearance of tapered lines being affected by their edges overlapping adjacent pixels, there
29
is also evidence of patches of various colours at several positions. As discussed previously it
is assumed that these result from the way in which the colour and intensity signals for each
individual pixel are derived from the signals for that pixel and from the adjacent pixels.
Based on the results of my Graphical simulation I concluded that for an image of a black
and white grid pattern the thickness of the lines at the limit of resolution would be about 1.5
pixels and the quality of the image would depend on the amount by which the edges of the
lines overlapped adjacent pixels.
For my Panasonic FZ20 and FZ50, the vertical resolutions of 1280 LPH and 1824 LPH
estimated by dividing the number of pixels in the picture heights of 1920 and 2736 pixels by
1.5 are fairly similar to the values of 1300 LPH and 1800 LPH published in the DPReview
reviews of these cameras.
If that conclusion is accurate then dividing the number of pixels in the height of the sensor by
the limit of resolution should give a value of 1.5. Suspecting that to be true for all digital
cameras including DSLRs, I decided to compare the sensor size and resolution data
published in the DPReview reviews of a range of cameras including four DSLRs which
covered a range of sensor sizes and resolution limits.
The results obtained are shown in above table in which they have been arranged in two
groups, each in ascending order of sensor size in MP. As will be seen from the table the
values obtained by dividing the height in pixels by 1.5 are very similar to the vertical
resolution values in LPH in most cases.
The largest deviation from the predicted value of 1.5 pixels/line is for the Canon EOS 1Ds
(DSLR) which with a value of 1.35 pixels/line is just 10% lower. That suggests that provided
the lens is of a high enough quality the resolution limit of a digital camera in LPH can be
estimated fairly accurately by dividing the height of its sensor in pixels by 1.5
Because of the effect of the edges of an image overlapping adjacent pixels the precise
details of an image of a resolution test chart captured in a resolution test will depend on the
exact location of the image in relation to the pixels on the sensor.
As illustrated in the Graphical simulation and in the stepped Es image the exact nature of
the image can be changed slightly by moving its edges by just a fraction of a pixel height or
width.
Taking the FZ50 as an example with a pixel size of 1.94 microns, i.e. less than one five
hundredth of a millimetre, that indicates the extreme precision which would be required in
order to successfully record two identical images, the second of which was recorded after
removing then replacing the camera.
30
As a result of removing and replacing the camera, differences could result from the camera
having been moved up or down or sideways by a fraction of a pixel or by tilting it by an angle
of less than the height of a pixel over the width of the sensor. It is even possible that due to
the tolerances in the focussing system, resetting the focus between two successive
exposures without moving the camera could result in slightly different versions of the
resolution test pattern when examined at pixel level.
Close examination at high magnification of the DPReview resolution test images for the
cameras listed in the above table revealed that they all looked very similar to the image from
my FZ50 shot of the ISO12233 based test chart in that they all exhibited the effect of the
edges of the lines overlapping adjacent pixels which is characteristic of digital images. In the
above table the data for the various cameras listed was copied from the reviews published
by DPReview http://www.dpreview.com/.
Also as previously stated, in addition to the appearance of the tapered lines being affected
by their edges overlapping adjacent pixels, when viewed at high magnification there is also
evidence of small patches of various colours at several positions in the enlarged section of
the image of the ISO 12233 resolution test chart. Although the colours and their intensities
varies from model to model, examination at high magnification of the DPReview resolution
test images for all the cameras listed above revealed small feint patches of colour.
Coloured images
In “FZ50 Resolution Test Results”, the variation of the resolution of the FZ50 with colour
was discussed in detail in relation to the Excel plots derived from the test results
Based on the ability to resolve the three horizontal lines of a letter E of various colours, the
results of the tests described in this document indicate that for the FZ50 the maximum
vertical resolution for the black, red and magenta letters is around 1950 LPH, with values of
around 1800 LPH for the blue and green letters, 1500 to 1700 LPH for the yellow and 1400
to 1600 LPH for the cyan letters.
From these values it is clear that the resolution of the FZ50 does vary significantly with
colour, confirming the suspicions which arose from the results of my FZ20 tests. The results
also indicate that the resolution is slightly lower with the lens at maximum aperture than
when it closed down by half a stop. Probably due to the effects of diffraction, they also show
a fairly steady reduction in resolution as the lens is progressively stopped down. However
there is no evidence of an optimum aperture or ‘sweet spot’ for any of the focal length used
in these tests.
As discussed in “Test chart design”, the most probable causes of the variation of resolution
with colour include the arrangement of the red, green and blue components in the Bayer
matrix and the way in which the colour and intensity data assigned to each individual pixel is
obtained from that pixel and the pixels which surround it.
Due to the differing effects of diffraction at different wavelengths the effect for each colour
will vary to some extent with aperture. In addition as the aperture is decreased the resulting
colour dependent increase in the diameter of the Airy disc due to diffraction may result in
edges in the details of an image overlapping an increased number of pixels thus reducing
the resolution of the various colours further and possibly to a different extent.
Part of the variation observed in the resolution tests described in this document may
however be due to differences in the relative intensities of the individual colours on the
printed chart, which would of course depend on the accuracy with which my Canon printer
printed the colours.
31
The results of these tests suggest that for a digital camera, the maximum resolution possible
is defined by the number of pixels in the height of the sensor. They also show that due to the
Bayer matrix and the effect of the edges of images overlapping adjacent pixels the resolution
will be colour dependent to some extent.
The maximum resolution achieved in practice will probably be somewhat lower due it to
being degraded by a range of factors. These include the physical size of the sensor which
affects the noise level, the performance of the signal acquisition electronics, the quality of
the demosaicing and image processing software, the User selectable settings such as ISO,
Contrast, Sharpness, Saturation, Noise Reduction, etc. and of course on the quality of the
lens, diffraction effects and any lens corrections performed by the camera software.
32
8. TC-E15ED Vignetting
Assuming that selecting the TC option in the set up menu of the FZ50 would result in the
camera’s OIS system, etc. being adjusted to match the characteristics of the Panasonic
DMW-LT55 1.7X teleconverter, I decided to buy a 1.7X Olympus TCON-17 teleconverter as
a less expensive alternative. Unfortunately I was somewhat disappointed by its performance.
In view of the responses to a thread I posted in the Panasonic Talk Forum of the DPReview
web site I decided to return the Olympus T-CON17 and buy a used Nikon TC-E15ED.
In order to accommodate the movement of the lens of the Nikon cameras for which it was
designed, there is a deep recess at the rear of the TC-E15ED. When attached to the lens of
the Panasonic FZ50 that additional space results in significant vignetting.
After reading on the PT Forum, about the possibility of shortening the TC-E15ED to reduce
the amount of vignetting, I decided to use my lathe to shorten mine and to cut a 55 mm
thread at its rear end to allow it to be screwed directly into the 55 mm filter thread of the
FZ50 as shown above. Subsequent measurements showed the gap between the rear face of
the rear element of the TC-E15ED and the front face of the front element of the FZ50 to be
2.5 mm.
In order to assess the extent to which the vignetting had been reduced by shortening the TC,
I assembled a spacer with which I could restore the TC to its original length.
Vignetting tests
To assess the amount of vignetting I took series of shots of a uniformly grey sky on a misty
day. To investigate the variation of vignetting with focal length I took additional shots at
various focal lengths with and without the spacer and without the TC.
The improvement which resulted from shortening the lens can be assessed visually by
comparing the following test images.
33
Vignetting Test Results – Images
Vignetting comparison at 420 mm focal length
Vignetting comparison at 300 mm focal length
Vignetting comparison at 200 mm focal length
Vignetting comparison at 135 mm focal length
34
Vignetting Test Results – Plots
The Silkypix image editing software provided with the FZ50 displays the values of the red,
green and blue components for the pixel at the co-ordinates of the cursor on the image.
Using Excel, it was possible to create a table in which to enter a set of co-ordinate positions
and the corresponding red, green and blue components.
The red, green and blue values for each diagonal step of 100 pixels (60 vertical and 80
horizontal) from the top left hand corner of the image were transferred to the Excel table for
each image. From these the combined brightness was calculated for each diagonal step. To
compensate to some extent for the very irregular nature of the pixel to pixel variation in
brightness, these results were normalised using the average of the brightness values for the
eight positions closest to the centre of the image.
The following images show the normalised brightness distribution in steps of 100 pixels from
the top left hand corner along the diagonal to just beyond the centre of the FZ50’s 3648 x
2736 pixel image.
Vignetting Comparison - No TC, Original TC, Shortened TC
Vignetting Comparison – Focal lengths - 420mm to 246 mm
35
Vignetting Comparison – Focal lengths - 420mm to 135 mm
EZ Zoom
The features of the FZ50 include the often misunderstood Extra Optical Zoom, EZ Zoom,
which is very popular with wild life photographers. With this feature, only the data from a
selected area at the centre of the sensor, commonly 5MP, is displayed in the viewfinder and
recorded in the SD card.
As this reduced area of the image is magnified to fill the EVF or LCD, it gives the illusion that
the focal length has been increased. Many users find that the use of EZ Zoom gives the
benefit of seeing the detail in their chosen subject more clearly. It is possible that it may also
allow more accurate metering and focussing.
However as the actual focal length of the lens is not increased, the resolution of the image is
not increased. Thus, apart from any potential differences due to more accurate focussing
and/or metering, exactly the same result would be obtained by cropping of a full size image
during post processing.
Even when the FZ50 is used at its maximum focal length, shortening a Nikon TC-E15ED TC
results in a significant reduction in vignetting, as was illustrated in the preceding images and
plots. It is also clear from these that the reduction in vignetting is significantly greater as the
focal length is reduced from 35 mm equivalent focal length 420 mm by zooming out.
However because of its popularity with wild life photographers I decided to assess the
potential benefits of using a shortened TC-E15ED in conjunction with an EZ setting of 5MP
as the focal length is reduced. The following images, which were shot using the popular 5
MP EZ Zoom setting illustrate the benefit of shortening the TC-E15ED in significantly
reducing the amount vignetting when zooming out.
36
In both of the following sets of images, those shot with the TC at its original length are in the
top row and those shot with the shortened version are in the bottom row.
Vignetting comparison - original length and shortened - 420, 304 and 200 mm
Vignetting comparison - original length and shortened - 137, 91 and 71 mm
37
9. Resolution Tests with Nikon Teleconverters
To assess the resolution of my FZ50 at maximum zoom with my Nikon TC-E15ED and Nikon
TC-E17ED teleconverters I carried out a series of tests with the final version of the test chart.
These tests were conducted as previously described in “FZ50 Resolution Tests” for the
tests at 35 mm, 135 mm and 420 mm.
Resolution at 420 mm with and without TCs
After I’d shortened my TC-E15ED to reduce vignetting I took a series of shots of the test
chart to assess the resolution of the FZ50 at 420 mm with the TC attached. Several months
later, I bought a used Nikon TC-E17ED and took a similar set of test shots to allow me to
compare the resolution of the FZ50 with the TC-E17ED with the previous results for the
FZ50 on its own and with the TC-E15ED attached.
For consistency all of these shots were taken as previously described for the tests with my
FZ50 at focal lengths of 35, 135 and 420 mm with no TC. To match the previous scale factor
of 0.25 mm/pixel the FZ50 was set to maximum zoom and the distance from the test chart
adjusted until the image of the 100 mm scale line occupied 400 pixels. Note that the required
distances of 14.43 metres for the TC-E15ED and 16.72 metres for the TC-E17ED are slightly
less than the 15.14 and 17.15 metres based on 10.09 metres multiplied by their
magnifications 1.5X and 1.7X which apply only when focussed at infinity.
As the scale factors for all of these shots were matched within a few percent, in spite of the
images being taken at different distances from the test chart, the resolution of those taken
with the TC-E15ED at 14.43 metres and with the TC-E17ED at 16.72 metres should in the
absence of any losses due to the TCs be identical to the resolution of the equivalent shots
taken from 10.09 metres without a TC.
After setting the white balance manually and turning off the OIS I used a remote shutter
release to take shots at 0.66 EV above the assessed exposure and at +/- 0.33EV. The
resulting images were assessed and analysed as discussed previously and the data derived
from them transferred to tables and plotted using Excel. The results obtained are shown in
the following images, in which the resolutions for black and each of the six colours are
plotted against aperture.
Resolution vs. Aperture – FZ50 at 420mm without a TC - Range 10.09 Metres
38
Resolution vs. Aperture – FZ50 at 420mm with TC-E15ED - Range 14.39 Metres
Resolution vs. Aperture – FZ50 at 420mm with TC-E17ED - Range 16.72 Metres
Comparisons for black and six colours
The following plots show for the FZ50 at a focal length of 420 mm without a TC, with the TCE15ED and the TC-E17ED the resolution versus aperture for black and for each of the six
colours.
Although derived from images which were taken from different distances from the test chart,
as these images have the same 0.25 mm/pixel scale factor, the resulting plots allow accurate
comparison of the differences in resolution for each of the individual colours for the FZ50 on
its own and with each of the two TCs. They also allow the differences for each of the colours
to be more easily seen.
39
40
41
10. Resolution with single and stacked teleconverters
Test method
Due to the high magnification and minimum focus range of the stacked combination of the
TC-E15ED and TC-E17ED and the limited space available it wasn’t possible the use of the
0.25 mm/pixel scale used for all the previous tests.
To allow the resolution for each lens teleconverter combination to be compared a series of
shots were taken from the longest possible fixed distance of 25.6 metres. The above view of
the test chart was taken from the same position with the focal length of the FZ50 set to 35
mm. Apart from the constant distance of 25.6 metres from the test chart the test procedure
followed was as described previously for the tests without a teleconverter. As previously,
after setting the white balance and exposure, +/- 0.33 EV shots were taken at half stop
intervals for each lens/TC combination.
Resolution test results - images
The following images are centre crops from F/4 shots for each lens/TC combination.
420 mm at F/4 from 25.6M with no TC
42
420 mm at F/4 from 25.6M with TC-E15ED
420 mm at F/4 from 25.6M with TC-E17ED
43
420 mm at F/4 from 25.6M with TC-E15ED and TC-E17ED
Resolution at 420 mm with and without TCs
As with the previous tests the assessed data was used to plot the resolution for black and
the six colours. As all of the shots were taken from the same distance the scale factor was
different for each lens combination. The number of pixels in the image of the100 mm scale
line and the 400 pixel value used previously were used to scale the results. Based on the
1.5X and 1.7X magnifications of the TCs and the 10.09 metre range with no TC the range for
the stacked TCs should be 25.73 metres. However for the stacked TCs at 25.6 metre range
the length of the image of the 100 mm scale line was only 343 pixels instead of 400 requiring
a scale correction of 1.17, 400/343.
For the shots taken without a teleconverter, the resolution from the distance of 25.6 metres
was so low that it was possible to distinguish only the 3.5 pixel Es printed in red, black and
magenta with all of the other colours being indistinguishable. With the exception of the shots
taken without the teleconverter, for which the Es were too small to allow them to be
assessed, the results derived from these tests for black and each of the six colours are
shown in the following images.
Resolution vs. aperture - 420 mm with TC-E15ED at 25.6M
44
Resolution vs. aperture - 420 mm with TC-E17ED at 25.6M
Resolution vs. aperture - 420 mm with TC-E15ED and TC-E17ED at 25.6M
Comparisons for black and six colours
The following plots show for black and each of the six colours, the variation of resolution with
aperture at a focal length of 420 mm for the FZ50 on its own and with the Nikon TC-E15ED
and TC-E17ED attached both individually and as a stacked combination.
In order to provide a consistent standard for these comparisons, with the exception of the
plot for the FZ50 with the stacked TCs, these show for each combination of lenses the
variation of resolution with aperture for the distance at which the 100 mm scale line on the
test chart has a length of 400 pixels in the image giving a standard scale factor of 0.25
mm/pixel, i.e. 10.09 metres, 14.43 metres and 16.72 metres respectively.
As it wasn’t possible to match the 0.25 mm/pixel scale for the shots taken with the stacked
TCs at 25.6 metres the plots for the stacked TCs have been corrected as previously
described to take account of the different scale factor.
45
46
47
11. Discussion - Teleconverters
Individual TCs
As with the previous tests with my FZ50 at focal lengths of 35, 135 and 420 mm, for the tests
with my TC-E15ED at 14.43 metres and TC-E17ED at 16.72 metres the scale was set to
0.25 mm per pixel by adjusting the distance from the camera to the test chart so that in the
test images the 100 mm long scale line had a length of 400 pixels within a few percent. Thus
for the shots taken with the TC-E15ED at 14.43 metres and the TC-E17ED at 16.72 metres
the resolution for all colours should ideally match that of the equivalent shots taken from
10.09 metres without a TC.
As is evident from the previous tests with the FZ50 at focal lengths of 35, 135 and 420 mm
there is a significant variation of the resolution with some colours. Although for these tests
without a TC the difference is not great, it is clear that the resolution is highest for the red
and black Es, with magenta and blue slightly lower, followed by green then by yellow and
cyan which shows the poorest resolution. As discussed in relation to the previous tests, due
to the difficulty of deciding on the limit of resolution in some images, there is some scatter in
the results.
As expected the results of these tests with the TCs show similar variations with colour
however as will be seen by comparing the plots for some colours there is some additional
loss of resolution.
TC-E15ED
Based on ratio of the 14.43 and 10.09 metre distances involved the level of detail which can
be resolved with the TC-E15ED attached would be expected to be higher by a factor of 1.43
than that from the FZ50 from the same position without the TC.
As will be seen from the plots, for the shots taken from 14.43 metres with the TC-E15ED
attached, the maximum resolution for black, red and magenta at the larger apertures has
been reduced from around 1950 LPH to around 1700 LPH, a factor of 1.15, with a less
significant reduction at the smaller apertures.
However, as the distance from the camera to the chart is greater by a factor of 1.43 the level
of detail which can be resolved is factor of 1.3 higher than would be expected for the FZ50
without the TC from a distance of 14.43 metres. Consequently it can be concluded that for
the same resolution the TC-E15ED has increased the ‘reach’ of the FZ50 by a factor 1.3.
For the larger apertures the drop for cyan is only from around 1520 LPH to 1450 LPH.
However for green it has dropped from between 1700 and 1800 LPH to around 1400 LPH.
For yellow there is an even greater drop from between 1500 and 1600 LPH to around 1100
LPH.
For all of these colours the reduction at the smaller apertures is less significant. That may be
due to the interaction of diffraction effects, the effect of the edges of the images overlapping
adjacent pixels and to the arrangement of the colours in the Bayer matrix. It is assumed that
for the larger apertures there are no diffraction effects so that the reduction is related solely
to the optical properties of the TC-E15ED.
48
TC-E17ED
Based on ratio of the 16.72 and 10.09 metre distances from the test chart the level of detail
which can be resolved with the TC-E17ED would be expected to be higher by a factor of
1.66 than that for the FZ50 from the same position without the TC.
As will be seen from the plots for all colours for the FZ50 at 420 mm with and without the TCE17ED, there is very little difference in the resolution for black, red, blue and magenta
except that for magenta the resolution is slightly lower for apertures larger than F/5.6. For
green the resolution is slightly reduced over the full range of apertures.
Although the resolution is slightly lower for cyan it hasn’t been reduced further by the TCE17ED. For apertures larger than F/8 there is a slight reduction for yellow which increases
with increasing aperture.
From these results it can be seen that with the TC-E17ED attached the level of detail which
can be resolved at a focal length of 420 mm will be about 1.66 times greater than from the
same distance without the TC. Consequently it can be concluded that for the same
resolution the TC-E17ED has increased the ‘reach’ of the FZ50 by a factor 1.66.
Plots for black and individual colours
The colour dependent differences in the resolution can be compared more easily by
examining the plots for black and individual colours.
As will be seen from the plots for the black Es, for apertures greater than F/8 the resolution
of the FZ50 with the TC-E17ED attached is only slightly lower than that of the FZ50 without a
TC and for apertures of F/8 and smaller it is identical. For the FZ50 with the TC-E15ED the
resolution is slightly lower than for the TC-E17ED over the full range of apertures.
In the plots for the six colours although for both the TC-E15ED and the TC-E17ED there is
some variation for red, blue, magenta and cyan. The most noticeable effect is for the TCE15ED for which there is a significant reduction in the resolution for green and an even
greater reduction for yellow which increases significantly with aperture.
Stacked TCs
For the FZ50 with stacked TCs due to the limited distance available and to the high
magnification and minimum focus range of the stacked combination of the TC-E15ED and
TC-E17ED it wasn’t possible the use of the 0.25 mm/pixel scale used for all the previous
tests. To allow a fair comparison with the other FZ50 lens and TC combinations a correction
factor based on the size in pixels of the image of the 100 mm scale line on the test chart was
used to scale the data for the plots.
In the images taken with stacked TCs the length of the 100 mm scale line is only 343 pixels,
86% of 400. To increase its length to 400 pixels would require the camera to be closer to the
test chart. As that was impossible due to the 25.6 metre minimum focus distance the
measured values were multiplied by 1.17, 400/343, to compensate for the reduced image
size and provide the corrected resolution values shown in the plots for the Stacked TCs.
As will be seen by comparing the plots of resolution versus aperture for black and six
colours, for black, red, blue and magenta the resolution versus aperture plots for the FZ50
with the stacked TCs are very similar to and only slightly lower than the corresponding
values for the FZ50 at 420 mm with no TC at a range of 10.09 metres.
49
For cyan the results are again similar though at a lower level for both. However for green
and even more so for yellow the resolution with the stacked TCs is significantly lower. As
similar results were obtained for the FZ50 with the TC-E15ED on its own this result was not
unexpected. The differences in resolution for black and each of the six colours can be more
clearly seen from the separate plots for each of these individual colours.
Because of the need to scale the results for the stacked TCs due to the problem with the
minimum focus distance it is more difficult to assess the extra reach provided by the stacked
TCs. However apart from the reductions in resolution for green and yellow the test results
suggest that the resolution of the FZ50 with stacked TCs from 25.6 metres is only slightly
lower than would be obtained at maximum zoom with no TC from a distance of 10.09
metres.
A similar comparison for the FZ50 with just the TC-E15ED or the TC-E17ED shows that
apart from the reductions in resolution for green and yellow in any combination which
includes the TC-E15ED the use of either or both of these Nikon TCs provides a significant
reach advantage over the FZ50 at its maximum zoom of 420 mm. In addition with the
shortened version of the TC-E15ED there is very little vignetting at maximum zoom and the
zoom level can be reduced considerably before it becomes significant.
50