Normal lens on cropped sensor camera and distortion [duplicate]

Question

I'm a novice on photography, so I hope I can explain myself clearly. I have a Nikon D5100 (cropped DX sensor) with kit lens, 18-55mm. I'd like to buy a normal prime lens, getting faster shutter speeds, greater aperture and overall better "normal" photo quality. Given my cropped sensor camera, a 35mm would probably be the best focal length to look for, as far as my research goes (50mm, some would argue).

And now it's when my speculation comes in: the distortion I get from moving away from normal focal lengths I suppose is mostly a factor of the focal length itself, not the sensor size. If I get a FX 10mm lens on my camera, it would give a 15mm field of view, but the fish eye distortion would remain the same, even if giving me just a smaller portion of the picture I'd get with a full frame sensor and hence not showing the more distorted outer edges, isn't it? Does that mean that with such 35mm DX lens I'll get a normal field of view and at the same time a slight fish eye distortion? And that a fully "normal" photo (field of view and minimal distortion) would only be obtained with a 50mm FX camera or a 50mm DX while standing far away from my subject?

Answer 1

There are two types of distortion:

1) Perspective distortion caused by being too close to your subject

2) Barrel or pincushion distortion caused by the lens optics

35mm and 50mm primes generally have very little distortion in the lens so any distortion you would see would be perspective distortion. You would tend to get more perspective distortion with the 35mm lens because you need to get closer to your subject with the 35mm than you would with a 50mm lens.

If you stand far enough away, both the 35mm and 50mm lenses will give you distortion free images. If you stand too close, both will give you perspective distortion.

This is why 50mm on a crop, and 85mm on a full frame, are often recommended as the minimum lengths for "portrait" lenses.

Answer 2

As Mike has already said, there is minimal technical distortion with lenses around 35-80mm. But to pick up on your other point, the "cropped" sensor of the D5100 will only take the image from the central part of a "full Frame" lens, which is similar to you taking the middle 67% of an image taken on a full frame camera. You are effectively cropping the outside of the image, not changing the focal length of the lens.