As far as I know the sensor of the D600 is the same as the D610, with minor improvements going into the D750, Z6, Z6II and D780. The new generations have higher ISO limits. Is there a noticeable improvement in the image quality, especially at low-medium ISO?
I mostly use AI lenses, the D600 viewfinder is not the best for manual focusing and I know the mirrorless cameras have better focusing aids which should help with focusing. IBIS would also be welcome. Lack of aperture information in EXIF data with manual lenses would be a disadvantage.
I like the long battery life of DSLRs, but USB charging should make the shorter battery life of mirrorless cameras less painful.
I was in the same situation as you. Bought the D600 on the day of its release, used and loved it. I use Ai and AiS lenses a lot, and have many of them, but manual focusing was difficult with some of them. I was often off by just a little (which in my type of phtotography may sometimes be ok, but at times the resulting image became sort of pointless and I just deleted it).
Then a little used D750 came my way and I used it for next to a year. One difference was very noticeable: the white dot for accurate focus was indeed accurate, and snappy. I used it with a DK-17M finder loupe on an adapter (and, mostly by luck, it never slid off or was lost). My keeper rate was much improved, and the image quality was noticeably better at higher ISO (must confess that I am both lazy and busy and shoot jpegs most of the time but the difference at ISOs above 6400 was remarkable. The images also seemed to hold slightly more detail. I believe these differences were mostly down to the in-camera processing, going from Expeed3 to 4)
But there were two things I was not very fond of: The shutter was clonky. And the jpegs were too contrasty and a bit difficult to tune back to normal.
What then happened was a sort of revelation. I picked up a very well used D810. Now, this camera is a beauty. The DK-17M sits snugly in place. The autofocus accuracy is perfect (just like the D750), and with a wider spread of focus points. I can focus quickly on the screen itself but having learnt to rely on the white dot I now use it to finely adjust somewhat difficult lenses like the 24/2,0 or 50/1,2. Even though I most often use these stopped down one stop, I can get accurate focus wide open.
Along with the D810 I was thrilled that it came with an unused adapted K3 screen. But the focus accuracy with the normal screen + white dot was so good that I never even bothered to try to install it.
Furthermore, one of the best things about the D810 - it is silent. I shoot in Q mode, and nobody notices. It is almost stealthy.
And the image quality with old lenses... there seems to be something to it. I don't know if it has anything to do with sensor stack or what it might be, but I never had the kind of image quality from my old lenses that I get now. I may be fooling myself, but the difference seems greater than what you might expect as a result of the slightly higher resolution and lack of anti-aliasing filter. Also despite the higher resolution, I have not had to increase shutter speeds. This body is heavier, the grip is good, it feels very stable and with little vibration. Also, it has the same Expeed 4 version as the D750 although high ISO is not as impressive as the D750.
The Df? I was not tempted for a number of reasons and never tried it. But I would guess that, because it shares the AF sensor unit of the D600, it might not have the white dot accuracy of the D810.
/Gabriel Heyman
