Now we see through a glass, darkly...
- Apr 5, 2016
When you crop a FF sensor, you throw away all of the "extra light" that was collected by the FF sensor due to its larger surface area. If the pixel pitch is the same and the technology is identical at the photosite (a/k/a pixel well) level, then there's absolutely zero difference between cropping the image from a FF sensor after the fact and using an APS-C sensor at the time of image capture.Crop sensors are good for reach. That said, when I crop full frame photos for "more reach" the resolution is always better, at least on a screen. Full frame sensors allow 2X the light of cropped sensors, have better DR and ISO performance. The 7D is/was a great camera, but it is no 5Dxxx in any manner other than perhaps a few more frames/second.
The reason most FF sensors have better DR and ISO performance is because their photosites are larger than most APS-C sensors, thus allowing more light to be captured per photosite.
But if one takes, for example, an EOS 5Ds (with a low pass filter) and compares it to a 7D Mark II where both have the same pixel pitch and use the same generation of technology, there's no difference between the two apart from autofocus.
The wider baseline allowed by the wider mirror in FF cameras affects the performance of the AF system with PDAF using reflex mirrors. The narrower baseline required by the narrower mirror in APS-C DSLRs negatively affects AF performance compared to FF DSLRs. But when talking about mirrorless, which does not use microlenses to redirect light from the edges of the lens to AF sensors in the center of the AF focus array the way PDAF systems using reflex mirrors do, the difference between FF and APS-C in AF performance is negligible. That's why the 90D has much better AF in Live View than when using the OVF.