A month ago I reported about a new Nikon D750 issue that is present when shooting backlit subjects – if the light source is just outside the upper parts of the frame, you get some strange internal reflection/flare issues (see those flickr samples: -1- | -2- | -3- | -4-). This issue has been documented and reported by many websites in the past few weeks:
A member of the Polish forum nikoniarze.pl found a possible solution (see detailed translation and pictures and also this thread) – you just need to place a small non-reflective plate/tape right in front of the mirror:
Red arrow – you can see the light reflecting off the AF sensors
Green arrow – curtain that does not (fully) hide the reflection when AF sensor is mounted too shallow
The blue arrow – shutter’s frame forming a shadow on reflection (flare)
Here is a video demonstration on how the problem disappears after covering the small gab under the mirror:
