In response to yesterday’s Nikon/Sony 70-200 testing, a reader commented:
Isn’t it established that Sony lenses are optimized for the Sony sensor and vice versa. (although more skewed against WA due to Sony’s sensor lenses and thickness. Putting the Nikon on the Sony is a little like putting P-Zero’s on a Prius.
I don’t believe that contention is established. In fact, if we’re talking Nikon F/Sony FE, I don’t think it’s true at all.
It is true that lenses are designed for a specific sensor stack thickness (thickness in this case refers to the combination of physical thickness and sensor stack index of refraction). Mismatches between the sensor stack thickness the lens was designed for and the sensor stack thickness of the camera in use can cause corner smear. However, this is only noticeable in cases where off-axis ray angles are such that the corners of the sensor is receiving light at angles that differ in material fashion from perpendicular.
From a practical point of view, this means that you need not worry about corner smear with lenses designed for SLR’s. The designers of those lenses, even if they were designing for the zero stack thickness of film, had to make the lens so that it would clear the flapping mirror, and that in almost all cases has the consequence of moving the exit pupil of the lens far enough from the imaging plane that even corner rays are sufficiently close to perpendicular.
That’s why no one complains about corner smear when film-era Nikon lenses are used on Nikon DSLRs.
It’s also why you can use high quality Nikon F-mount modern lenses like the Zeiss Otus 55/1.4 and 85/1.4, the Zeiss Distagon 15/2.8 DF.2, and the Zeiss 135/2 Apo-Sonnar ZF.2 on Nikon and Sony alpha cameras and get great results on both. The same is true off Leica R-mount lenses.
The other issue with non-perpendicular rays is corner color casting. There are two relevant practicalities here. The first is that the same mirror-avoidant lens designs that make sensor stack a non-issue for SLR lenses also keep material corner color casts at bay. The second thing to keep in mind is that the a7RII back-side illuminated (BSI) sensor and color filter array (CFA) design make it immune to corner color casting even with rangefinder lenses.
The final salient issue is that the Nikon DSLR and the Sony alpha sensor stack thicknesses don’t appear to be that far apart.
For all the above reasons, Nikon F lenses do very well on Sony alpha 7 cameras.
However, there is one thing to consider when doing lens testing: the raw developer. Lightroom looks at the lens data and applies sharpening based on that, which can tilt the playing field in confounding ways. The solution is to use a dumber raw developer such as DCRAW. I didn’t do that yesterday, and that was less-than-perfect experiment design. Eyeballing the images, I don’t see any tell-tale signs of differential sharpening, but I would be remiss if I didn’t point this possibility for error out.