Just arrived from Kolarivision is a Sony A7R in which the sensor cover glass has been replaced with significantly less thick glass. See A Thinner Sensor Stack at LensRentals.com for some background info.
I’ll be shooting this modified camera this weekend, with a report to follow.
The thick sensor cover glass has meant that rangefinder lenses (Leica M, Zeiss ZM, Voigtlander) have had severely degraded performance outside (roughly) the central 1/2 of the frame, due to ray angle, as reported on for years, and documented extensively on this site.
General issues from sensor glass
- The main impact of an added glass plate to more oblique rays is the introduction of severe astigmatism in the imaging field. The tangential image surface is shifted forward while the sagittal is minimally affected.
- Astigmatism means a highly non-symmetric point image, not just being softer but killing the similarity between object and image structures. Often the blur resembles the effects of motion blur which can produce “cross-eyed” or double-image bokeh (very unappealing to the eye, downright ugly).
- On axis the added plate changes the correction towards ‘spherically overcorrected’ when the lens speed is fast, which produces less pleasing background bokeh.
- In general the sensor cover glass plate shifts the position of the image by about a third of the plate thickness – and that is the point where the dispersion of the plate itself comes into the game, since in the imaging field with oblique rays a longitudinal shift is always connected to a lateral shift. This effect modifies the lateral chromatic correction of the lens.
- Guide to Leica shows a number of examples of Leica M lenses badly degraded like this, such as the 18/3.8 Super-Elmar-M ASPH (look for camera name A7R).
- Astigmatism and Lateral Color: Why Performance is Lens + Sensor (aka why do my expensive lenses suck on mirrorless)