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This may be a silly question, however, I have seen a number of threads about various camera bodies from various manufacturers extolling the benefits of having the option of shooting in full frame mode or flipping a switch to then shoot in crop mode.  This is presented as an advantage of having the "reach" of a crop body when you need it.  My question is why not just shoot in full frame and then crop in post?  If you have, say a 45 MP FF camera and then shoot in crop mode doesn't the camera just use about 20 MP of the sensor?  So, couldn't you just crop a FF image to 50% of original and get the same image and detail?  Maybe I am missing something?

You are correct that you can just crop the FF image but there are potentially some advantages if you don't need 45mp and they can vary by camera.  Some of these include a much faster clearing of the buffer and a more than doubling of the buffer.  Some cameras can shoot at a faster frame rate.  Of course you are using much less card space and storage space on your computer if that is a consideration.  The point is you have the option of taking advantage of those items or not.  Some cameras also have other form factors such as 5x4, 4x3, 16x9, 1x1 or an APS-H crop (1.25x).  Many landscape photographers prefer other form factors and rather than crop later they do it in camera.  It's all about choice.  In my case, I shoot landscapes in maximum pixel mode and crop later but a lot of wildlife, especially birds I do shoot in APS-C partly for the buffer increase and partly because I simply never need that many pixels for a wildlife shot where I can't get enough of them for a landscape shot.  In my business, wildlife shots are almost always purchased by publications where landscape shots are usually purchased as a large print.

Just as an aside, shooting at 1.5x crop results in a 2.25 (1.5 x 1.5) increase in speed and buffer and a 1/2.25 reduction in pixels or 0.444x.  For Canon it would be 2.56 (1.6 x 1.6).

Thanks as always E.J. I appreciate the detailed response. I learn something new on this site all the time.

Ed Cordes wrote:Thanks as always E.J.  I appreciate the detailed response.  I learn something new on this site all the time.

My pleasure

I should also say that I shoot with a 61mp camera so when I shift down to APS-C it is still 26mp - still a massive amount of pixels in my bird shots.  A 45mp camera like the D850 ends up close to 20mp in APS-C mode.