Originally posted by bossa The D7100 has it's own 1.3x crop mode (just as the D800 has a 1.5x crop mode) which works out to be 1.3*1.5 (1.95) of a FF sensor. You end up with a 15MP file size and an effective 2x crop factor. A cheap $1200 300mm lens suddenly has the FOV (and a usefull file size) of an expensive 600mm lens.
It is irrelevant that an Xmm lens can have the same field of view as a 2Xmm,
if you've done that by throwing useful pixels away! If you don't do that extra 1.3x crop, the centre part of the recorded image has exactly the same field of view as the longer lens anyway - but
in addition it is surrounded by extra pixels that might be useful.
(I assume no one would say "if we do an extra 10x crop we will get the same field of view as a lens of 10x the focal length, so let's do it"! Yet that appears to be the same logic as that extra 1.3x crop).
Once a lens's image circle has covered the sensor, you might as well use all the pixels, unless you have composed carefully in the camera. If just a 1.5x crop means that all the pixels on the sensor are used for image formed by the lens, why do an extra 1.3x crop? It won't improve image quality for anyone using all the captured pixels. (I shoot DNG).