Hi,
Jim Kasson does some very intelligent testing. The way he tests, he uses a Stackshot for making a large set of exposures passing trough maximum focus and measures MTF for the RGB channels. That gives a lot of interesting information about:
- Longitudional chromatic aberration
- Focus shift when stopping down
- Effects of diffraction
Sony has a seemingly very good 90/2.8 macro lens, let's see how capable it is at optimum aperture:
We can see that the lens is very sharp, it reaches around 1800 lp/PH (line pairs per picture height). There is some LoCA, quite small as we will see.
Repeating the experiment at different apertures we can see the effect of focus shift and also the effect of diffraction.
What we see here is that focus shift is virtually none. Regarding diffraction we can see that resolution went from around 1700 lp/mm to around 1000 lp/mm. If we assume that 1700 lp/mm corresponds to 42 MP, at f/11 we would have something like:
42*(1000/1700)^2 ->14.5 MP
Now, lp/PH is normally measured at 50% MTF. Much of what is lost can be regained with sharpening, but sharpening can create artificial detail.
Now, let's look at a famous lens from long ago, the Leica 100/2.8 APO Macro
Peak performance is at f/5.6 and quiet a bit below the Sony lens. LoCA is wider spread, indicating that the Leica lens is not fully corrected for axial chroma for the RGB wavelengths of the Sony sensor. Something like 1400 lp/PH is measured.
Stopping down the Leica lens to f/11 yields around 1000 lp/PH, corresponding to f/11 on the Sony lens. But, the Leica lens has some significant focus shift. Focusing at full aperture would, it would achieve 700 lp/PH at f/11.
The last part of Jim's test shows that all lenses pretty much converge at f/8 and beyond.
In this measurement, the Sony is the sharpest lens, so it has most to loose.
Now, let's look at a real 'champ', the Fuji GFX 120/4 macro at f/5.6:
This is an impressive piece of a lens. LoCA is nil, and it reaches 2800 lp/PH. It i a bit helped by the aspect ratio of the GFX, but truly impressive, anyway.
Jim also tested the Voigtlander APO Lanthar 65/2 APO. An impressive lens that I will probably buy:
It reaches maximum performance at f/2.8, at around 2300 lp/PH on the Sony A7rII.
The focus shift/diffraction plot also indicates very little focus shift. It seems to quiet perform a bit better than the other lenses at f/11, that may be due to physical aperture being a bit larger. Macro lenses have floating elements and vary both focal length and aperture with focusing.
I would say that Jim's posting illustrate a lot of interesting phenomena. It may be that we don't see much of these in real world images. Why? Some explanations:
- Medium apertures are often used
- Focusing may be not accurate enough
- Observation may not be sensitive enough
Also needs to be said that Jim's data is on axis. Most lenses perform weaker of axis, but off axis measurements are much more difficult.
Jim has developed a protocol for qualitative off axis measurements, involving large distance and large targets.
Links:
http://blog.kasson.com/the-last-word/towards-a-macro-mtf-test-protocol/http://blog.kasson.com/the-last-word/focus-shift-and-loca-in-the-leica-r-1002-8-apo-macro/http://blog.kasson.com/the-last-word/focus-shift-and-loca-in-the-zeiss-1002-makro-planar/http://blog.kasson.com/the-last-word/focus-shift-and-loca-in-the-sony-902-8-fe-macro/http://blog.kasson.com/the-last-word/focus-shift-and-loca-in-the-nikon-1052-8-macro/http://blog.kasson.com/the-last-word/macro-on-axis-sharpness-and-loca-summary/Best regards
Erik