I'm looking for a 50mm lens for my nikon d3100 and I just came across an AF-S 50mm f/1.8G lens and it says in the description that if it's used on a DX camera, the angle of view is going to be 75mm. I don't understand... I want my angle of view to be around 50mm, not 75, but I was also looking for something that will autofocus on my camera. Can someone explain this?
Imagine you have an FX camera (or old film camera) with a 50mm lens, and take a picture. Then in post processing, you crop out the edges. You would still have an image with the same perspective of the 50mm lens, but by cropping you've effectively zoomed in on the subject/middle of your image.
This is basically what happens with a cropped sensor. Same 50mm lens, same image projected onto the sensor, but the sensor being smaller (DX), you only get a smaller part of that projection.
So the perspective and DOF is like a 50mm lens, but the subject area you get is roughly like having a 75mm lens (on a film or FX body) due to the zooming/cropping effect.
As an example, if you are shooting a full body shot of a person, or a statue in a museum, film camera and 50mm lens, and then switch to a DX body and 50mm lens, you will have to stand further back in order to get the same framing. If you want to be the same distance as you were with your FX + 50mm lens, you'd need something like a 35mm lens on a DX body.
No. A 50mm lens will always be a 50mm lens, irrespective of the size of the recording medium.
That said, the scene recorded on a smaller medium will have a narrower angle of view, which for an APS-C size sensor equates to an image that has about the same viewport as that produced by a lens with a 1/3 smaller focal length on a 35mm film frame.
Everything else (especially depth of field and telephoto compression) is however completely unchanged.
If you are looking for cheap autofocus 50mm lens for your DX camera, the Nikon 35mm 1.8G is very good, and is a 50mm equivalent on DX (35mm * 1.5 = 52.5mm)