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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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enter image description here

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.

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    It's true that cropping won't change the perspective, but if you stand in the same place (so the subject is smaller), crop to the same field of view, and enlarge, that will.
    – mattdm
    May 6, 2014 at 19:48
  • I'd say 135mm looks best there. Is it FF? Cropping will give same perspective as the eq. focal length as you stand back to teh same distance as the eq. focal length to get the same frame , and thus same perspective. but you destroy the nice DOF effect in doing so.
    – Michael Nielsen
    May 6, 2014 at 21:38
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    @dav1dsm1th Sorry, that was a little unclear. The cropped image and the image from a longer focal length with the same framing will have the same perspective, and therefore will have the same "effect". Given enough detail for the image/print size, they'll be basically indistinguishable.
    – mattdm
    May 6, 2014 at 23:02
  • No, I said you cant change position. Oh I get it. I was reacting to "Cropping a wide angle image to a longer equivalent will not remove this effect." Which indicates that you are saying that the long noses will still be there if you crop it. and I pointed out that to crop it , you will be standing the same place as you would shoot the longer focal length shot from . and thus "this effect" is gone. But the images are not equal. Your background is suddenly not to blurred (if you intended that, which is normally true).
    – Michael Nielsen
    May 7, 2014 at 10:34
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    In order for the depth of field to be roughly as shallow in this situation, you need to increase (open) the aperture by the crop factor. That is, to get the same depth of field as 50mm f/2.8, on APS-C you need (approximately) 35mm f/1.8. Of course, this assumes a number of ideal things, including sufficient resolution.
    – mattdm
    May 7, 2014 at 10:44
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It's not preferable to shoot a 50mm for portraits. It may be the preference for some, but others (including myself) prefer longer focal lengths, between 85-200 or more. Wider angle lenses have greater distortion. Cropping can be done of course, but the distortion by comparison is still present.

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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).

Which focal-length lens is usually used for portrait photography, and why?

50mm vs 85mm for portraits on a crop sensor?

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:

Is digital zoom really useful?

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.

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