Zeiss Batis 85mm f/1.8: the Negative Sharpness and Micro Contrast Effects of Correcting nearly 3% Pincushion Distortion
As context for my comments on the Panasonic LUMIX S 24-105mm f/4 coming soon, I was looking around for a good example of how distortion correction inevitably degrades micro contrast in areas that must be stretched, correction being particularly noxious for lenses with significant pincushion distortion because the central 2/3 of the frame gets stretched-out the most.
Distortion correction is relevant for anyone contemplating use of pixel shift or HighRes mode—the benefits are undermined by pixel-stretching.
With its 3% pincushion distortion, the Zeiss Batis 85mm f/1.8 is a prominent example of a decision to cheat on optical correction and instead rely on distortion correction*. Never a plus, distortion correction is demonstrably damaging to micro contrast, and especially in regards to future higher resolution cameras. Allowing it is a design decision that I see as relegating a lens to guaranteed 2nd-tier status by needing to correct its distortion. A particularly startling example is the extreme barrel distortion of the Leica Q, which guarantees weak outer zones (and makes a 47MP LeicaQ2 a bit of a joke in terms of sharpness potential).
Includes corrected and uncorrected images up to full camera resolution, with crops.
* As a related concern, I object to publishing MTF charts for raw optical performance when the lens flags itself as requiring distortion correction. The MTF chart is thus a best case that is never achieved with distortion correction.