For shooting portraits, why is 50mm preferable? For example with an APS-C sensor, why is it not ok to take photos with 35mm equivalent and simply crop them (to 50mm equivalent)? or take photos closer to the subject?
What's the difference?
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Notice the different perspectives in the image above, for lenses of differing focal lengths, caused by the distance between the camera and subject having to be changed - to keep the ratio of subject size to image size similar in each example.
Cropping a wide angle image to a longer equivalent will not remove this effect.
Moving closer, with a wider lens, will introduce more of this effect.
Cropping will also not affect the DoF in the image - where changing the lens and shooting position probably would have.
The original image (which is not mine) can be found here.
you question has one premise and 2 questions to answer:
P1. Premise: Why prefer 50mm?
We dont. 50mm on crop camera is just about a bit too short. Optimal is 85mm (crop) and 135mm (FF). A good range to experiment within (for beauty - and not fucked up funny perspectives) is 60-135mm (crop) and 85-200mm (FF).
Q1. digital zoom (cropping to simulate the focal length)
It's a nice workaround, but you will destroy your resolution and better optical properties and DOF:
Q2. Go closer instead
The reason there is a focal length recommendation is to get a certain perspective that is flattering. So getting closer is the worst thing you can do.