It wasn't long after the launch of the D750 that users starting reporting strange flare effects. After a while, it became clear they were all talking about the same phenomenon – diffuse, irregularly shaped light patches near the top of the frame.
It was soon obvious that this happens when there is a bright light source just outside the frame at the top of the picture. Normally you'd blame this on lens flare, but in this instance there is a narrow strip just above the flare spot which is perfectly OK.
In fact it's not caused by lens flare at all. It's an internal reflection from the AF sensor at the base of the mirror box and just in front of the sensor itself. See the red arrow in the picture above.
You won't see the offending component with the mirror in its normal position – the mirror needs to be raised first, just as it is at the moment of exposure. When a bright light hits this component at just the right (or wrong!) angle, it reflects up on to the sensor surface.
Nikon's free fix
Nikon's been making cameras for decades, so this is rather a strange issue for a company with that much experience. Failing to safeguard against internal reflections is a bit of a schoolboy error for a camera maker, though in Nikon's defence it does seem that the problem happens only in very specific circumstances – we didn't see any sign of it during our time with the camera.
Nikon USA has issued a service advisory saying it will fix cameras, free of charge, from the end of January. This involves moving the AF sensor and 'inspecting and service' the light shielding.
In the meantime, some D750 users have resorted to home-made masking solutions to fix the problem.
• Read our Nikon D750 review.