The Df has become my main camera since I bought it, in 2014. The D800 does the backup.
It is my nearly daily companion, while commuting, strolling, or travelling, and also finds uses at home.
I mostly use it with MF lenses (Nikkor, Zeiss, Voigtländer, Leitz R), because I mostly use such lenses in the first place. Exceptions are mostly the AFS 85/1.4 and the 300 PF, not to forget the very nice Tamron 45/1.8.
For MF, I did not feel the need for replacing the ground glass, but I've been systematically using the DK 17 M magnifying eyepiece from 2009, wfirst with the D700, then the D800.
I am especially delighted by its IQ; colors feel more natural than with the D800. Very good handling with only a few minuses (exposure compensation control is a bit awkward, while on the contrary shutter mode is easily knocked off position; AF coverage zone is too narrow; I never use the front wheel, which hurts my finger).
It needed servicing only once, in 2017 (aperture lever actuator became weak, then stopped working altogether).
The Nikon Z-something may replace my Df in the future, as I have tested them and was delighted by the results - but not so much by the handling, which is "only" fine. I may have to switch if my eyesight weakens or my hands get less steady, something that may happen also to me (I'm 62 after all). For the time being, I'm fully satisfied. You may have noticed that all pics I've been publishing these recent years, months, days, come from the Df.
