I am mostly a bird photographer so always looking for more reach. Now, if I used a D7100 with my 300mm + a 1.4 TC (420mm) would I have the equivalent of 630mm at 24 MP? That sounds like a better birder than the same lens combination with my D800, or would it be?
If the same lens and location is used, than yes because that the subject fills the frame with more pixels defining the subject. If the subject filled the frame on both cameras more pixels would defining the subject with the D800 Stan St Petersburg Russia
Depends. To see the 1/2 pixel diameter difference everything has to be optimum, rigid mount, high resolving power lens, under the point of diffraction limiting which starts at f/8 on a D800 and lower on a D7100. Theoretically, yes, it will be sharper but many things get in the way of that in practical field results. Again, if optimum distance needed to fill the frame on both, than the D800 will have higher resolving power.
None of this makes any difference in the overall impression of the image, both are capable of stunning as well as boring images Stan St Petersburg Russia