I wonder if anyone can offer some assistance. Something appears to have gone wrong with my D610, which is nearly 6 months old with approximately 7000 shutter activations. I have been using it this week without problem, and this morning I decided to do the long overdue AF fine-tuning on my lenses. Everything went OK with the first lens. I put a different lens on the camera and looked through the viewfinder to see everything was really dark as though there were a filter on the lens (which there was not). I could find no reason for this, and although I could just about see through it there did not seem to be enough light getting through for the AF system to work. So I changed back to the previous lens, only the find the problem was still there. I have just cleaned my sensor, but the camera was working fine post clean and worked fine this morning until I changed lens. I cannot see how this could have affected the light coming entering the camera from the lens.
Has anyone got any idea what could be going on here, or what investigations I should carry out?