Most rangefinder lenses used on a Sony a7 series camera show some serious issues because of the rather thick filter stack in front of the sensor. There are two ways to deal with this. We have previously discussed the use of front filters to counter the induced field curvature. In this article Sebboh reviews another solution.
This is an extended use review of the Kolari Ultra Thin (UT, also sometimes referred to as version 4) sensor stack modification on a Sony A7. This modification removes the AA filter and IR cut filter from a stock Sony camera and replaces it with an ultra thin 0.2mm thick IR cut filter in order to attempt to provide similar levels of performance to the Leica M9 in dealing with the steep ray angles often produced by rangefinder lenses designed for film. I have shot a UT modified Sony A7 for 6 months now and can confirm that it dramatically improves performance with certain lenses (big thanks to Nehemiah for letting me use his lenses and cameras for the comparisons shown here). I will try to outline what kind of improvements can be expected here and what drawbacks there are to getting the conversion done.