If you place a filter between the lens and the image plane you will introduce aberrations, particularly astigmatism. The problem is worse for short exit pupil distances common for Leica M lenses. I've seen the issue alluded to in Leica's press announcement regarding the IR problem, but I've never seen any meaningful discussion in any forum. So, I analyzed what would happen to a perfect f/2 lens if you put various thickness filters near the image plane. The chart below shows MTF at 40 cycles/mm for filter thicknesses of 0, 0.5mm, and 2.0mm. I think the damage is alot bigger than most people realize. Leica state that the filter they use over their sensor is 0.5mm thick. In addition, all sensors have a cover glass that ranges from 0.5mm to 1.0mm in thickness, so the actual total thickness is getting into the uncomfortable range. Of course, if you stop down the problem gets reduced substantially, but still, if you buy the latest 50/1.4 ASPH do you really want its performance degraded to such an extent? There has been much talk from various manufacturers in recent years about "designed for digital" lenses. A lens truly designed for digital must be corrected for a nominal filter glass near the image plane, but, strangely, nobody ever mentions this.