What you shoot at 2000 on the 7100, you will be able to push to 10,000 on the 750. You're gonna love it (except for lack of reach).I love that I can push my ISO's on the D7100 without too much image degradation or noise in low light situations. This Altamira Oriole was in the shade under the trees. I'm waiting right now for my D750 but plan to continue using the D7100 on birds with a long lens.
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Thanks and Really looking forward to my 750! Plan to use it extensively with my 105 Micro and for butterflies and dragonflies.
Welcome!