(Geek alert)
Following an earlier
thread on lens vs sensor resolution, I learned of Bart van der Wolf's
test image and some interesting
discussion of images of it from various situations. Since it is so easy to use (it is not "distance sensitive") I decided to give it a go and report anything interesting. There have been a lot of discussions of 7D vs 5D2, with a lot of "noise" and I have both cameras. What follows is in the format of a techincal report (that was a skill developed in my non-photography related career
) It was a fun experience for me, a bit like some of my scientific work from before I retired, so I hope someone finds it interesting - or have some suggestions for improving technique - I'm getting a bit behind the times on some of these things.
Cheers
Andy
Comparing resolution limits of 7D and 5D2.SummaryBoth cameras showed "true" resolution to about 92% of Nyquist frequency. Adobe Camera Raw provides that resolution (107 cy/mm for 7D and 71 cy/mm for 5D2), while DPP gives only 83% but notably better color moire control. For fabric/fashion, repeating things (distant bricks etc) DPP might be preferred, while for landscape and fine, irregular detail, ACR might be preferred. (I've not used any other converters) 5D2's AA filter seems to be a little weaker, but this did not seem to affect the “limiting on-sensor” resolution.
BackgroundI don't have Windows (neither on a PC nor on an emulator on my Mac), so I can't use Imatest – I volunteered long ago as an alpha tester for Mac but Norm never seemed to proceed with that side of the project. Bart's star seemed like a fairly easy way to test things for myself..
MethodsI printed Bart's “Siemens star” on Epson premium luster paper on my 7600 and all but the central 8-9 mm diameter was “resolved” - with moire artifacts and grey inside that. I added it to my aging test target close to the center, inside the yellow rectangle.
I shot at f/8 with a 7D and a 5D2 from about 20 feet using a 100mm f/2.8 L macro with IS off. Focusing was manual with Live View at 10X – the star made that very easy – tripod, 2 sec timer and remote release (mirror already up). Lens optical axis was at the height of the center of target and they were essentially at right angles - camera double bubble leveled, target vertical. Even with a 4 µm CoC (~the 7D pixel size) the DoF should be 6” (±3”) and inspection of all the focus attempts showed they were indeed indistinguishable. Raw files were converted by ACR (5.7 I think - in CS4, no sharpening) or DPP at “default settings” (in DPP all neutral, sharpness zero DPP 3.8.2 ). WB was not an area of concern so “as shot” was accepted for both. In PS CS4, white point and black points were set in a curves layer using the ends of the step wedge. My typical “sharpening” is a “1 pixel radius “Generic” deblur in
Topaz In Focus” and this was applied to all images. (The blur circles didn't change as a result of this). Composited images were converted to 8bit, sRGB and saved for web at 80% (this preserved all the relevant moire details). One image (1100x811 pixels) contains the original pixels from each camera, so the size of the images is different on the screen, consistent with the pixel size difference
The other image has the 7D image downsized (simple bicubic) to the same size as the 5D2's image, for those who like equal area comparisons. (It is 850x535 pixels) How one might uprez the 5D2 image to the same "size" as the 7D raises its own issues which I decided not to address
.
ObservationsIt took a bit of experience to place the right size of circle to define the “central blur” area and it remains a bit subjective, but I think I got to be internally quite consistent. All the radii hit the circle even if there is some evidence of aliasing outside the circle at that point. Some radii go inside the circle and that varies by angular position (not surprising given a fixed array and asymmetrical lens aberrations) but mostly it was either extreme moire/aliasing or just blurred. These may be easier to assess when viewed at 200 or 300% on your monitor. I guess the precision of the diameter of the circles I created would be ±2-3 pixels.
The cool thing about Bart's star is that there is no need to fuss over precise distances and focusing is a dream. The key pieces of information (for me) from this comparison
1) The confirmation of an oft-asserted but rarely-demonstrated difference in the AA filter between the 5D2 and the 7D. I had always been skeptical of such claims but this set of actual data seemed to support it. Looking at the star from the 5D2 it is clear that there is false information outside the circle (i.e. not just the radii but quite a lot of weird lines – aliasing coming down below Nyquist to irreparably, if subtly, damage the real data) and the clarity of the aliasing and the intensity of the colors inside the blur circle all point to a weaker AA filter – by how much, who knows?! The other indicator is the vertical line of 12 pt type from the ACR/5D2 data where the cyan/orange repeat is actually quite “pretty”! Some aliasing is also seen in the 9 pt type from the 7D, with the same color repeat pattern. While the AA filter seems somewhat weaker, based on the moire patterns and clear aliasing below Nyquist, it seems that the overall resolution limits of the two sensors are similar, when related to their own Nyquist frequency.
2) The raw converters make different compromises between perceived intensity of residual color aliasing and luminance resolution. ACR gave 91 and 92% of pixel-pitch-based Nyquist (107 and 71 compared to 116 and 78 cy/mm for the pitch of 7D and 5D2 respectively) while DPP gave 83% for both. Based on the earlier discussions, it's likely that this resolution limit represents around 10% MTF – although the comparisons rely on the consistency rather than absolute value. The ACR images had more residual color patterns close to the limit than the DPP ones. I know nothing about the inner workings of the demosaicing and color interpolations and the compromises that are presumably involved, so someone else will hopefully be able to comment.
3) The confirmation that the lens I used seems to outresolve the sensor – why I originally got started on this little project to satisfy my curiosity. (Looking at the star placed at different distances from the optical axis will await a rainy day).