Actually isn't D800 actually saving money on lenses with x1,2 and x1,5 modes ?
Yes. No. It depends:
- Yes, you can use your DX lenses on the D800, and still get slightly less than D7000 MPs (about 15 vs. 16). This, however, in a larger, heavier package, at about three times the price (but with some improved features), and with arguably decreased usability (DX mode in the FX viewfinder).
- No, if you want to pull the best performance out ouf the D800, you will need to use the full 36MP frame - that means FX lenses. And you will need to carefully select them - not every old one found in your cupboard will be up to this task.
- So, it depends - DX lenses on the D800 make sense as a migratory path, and where you want to keep using a seldomly used specialty lens (e.g. 10.5 fish eye) instead of "upgrading" that to FX (assuming that you have DX lenses in the fist place).
Fortunately, I have a stash of old Ai/AF FX lenses (most from the 70ies to 90ies, some still current models), that work perfectly well on the D700 (so far as I have already tested them in the last two days). So, keeping to use my three DX lenses is not a selling point to me.