(This post is from an email conversation with a coworker. I thought the content might be useful for others.)
Q: I currently own a Nikon D7100 and was wondering if I need to upgrade to a full frame Nikon D750? What are some of the advantages of full frame over my D7100? I currently own 3 lenses:
Nikon 18-200
Nikon 105MM macro
Nikon 10-24 wide angle lens
I am being told that all 3 lenses will fit my full frame D750. I thought about D810 too but all the online reading does not justify extra $1000 for D810 versus D750.
A: It would be helpful to know what kinds of shooting you do. I suggest it would take some pretty compelling reasons to recommend a move to a full frame body, since you really don’t want to use DX lenses on a full-frame body as that mostly defeats the purpose. If you go full frame, you’ll need to price in new FX lenses to go along with the body. I doubt your 105 will cover all your needs.
These are definitely generalizations, but full frame gives you better wide-angle options, better build, and lower noise at high ISO. The down side is expense, weight, size, and file size. I had to buy a complete new set of bags when I upgraded from DX to FX. On the other hand, in addition to the smaller size, weight, and cost of the DX gear, for some types of subjects – mainly wildlife / birding and sometimes sports – DX bodies can be better since the “crop factor” extends the reach of expensive telephotos.
I shot film for nearly 3 decades before moving to digital, so my early DX cameras felt a bit off due to the smaller sensors. I most missed the wide angle options and also had to get used to the depth of field differences. As soon as there was a pro-sumer full frame body I bought it (D700) and really enjoyed the difference. That said, I still have a place for DX in my shooting. Whenever I am birding I nearly always switch my D810 to DX crop for speed and to reduce the file size since seldom am I close enough to fill the frame, even at 600mm.
Anyway, a sense for your subjects would help make a more specific recommendation.