I am about to purchase a 35mm (Nikon 35mm F1.8 DX) for my APS-C camera, to get the equivalent of a 50mm lens. I want to use it for portraiture a bit, but I'm worried the facial deformations will equate to a 35mm lens (which would make it not that useful for portraiture).
Regards.
Short answer no - it will be approximately equivalent to the 50mm lens on a full frame camera.
What you are referring to are issues of perspective. The perspective is not a property of the lens but is due to the position of the camera relative to the subject. If you are at the same distance from the subject you will get the same perspective no matter what lens you get.
The focal length of the lens affects the magnification of the image and will thus change how much of the scene you get on the image. The sensor size also affects the magnification. A 35mm lens on an ASPC size sensor has approximately the same magnification as a 50mm on a full frame sensor.
In your case you will be at the same distance to the subject with a 35mm lens on an ASPC crop as you would be with a 50mm lens on a full frame to get the same field of view and thus the perspective will be the same.
The facial deformations you are worried about are due to perspective distortion. Perspective is determined by one thing and one thing only: subject distance.
If you take a picture with a 50mm lens on a FF camera from a distance of 10 feet and also take a picture with a 30-35mm lens on an APS-C camera from a distance of 10 feet both pictures will have the same perspective and framing (allowing for the rounding of focal lengths - a 50mm lens may be anywhere from about 46-53mm in actual focal length and most other lenses are also usually rounded to the nearest common focal length).
The focal length of a 35mm lens is always 35mm. A 35mm FX lens and a 35mm DX lens will both provide the same field of view on a camera with DX sensor (Again, allowing for that pesky rounding - all 35mm lenses aren't exactly 35mm. Some are 37mm, others might be 33mm).
The only reason we use crop factors is to compare the field of view (FoV) of lenses used on cameras with smaller sensors to the focal length needed to obtain the same field of view on a full frame body. When comparing the FoV of a 35mm DX lens or 35mm FX lens used on a DX body to the focal length needed for the same FoV on an FX camera both lenses need to be multiplied by the DX crop factor.
