Originally Posted by
GrumpyDiver
I personally find a 50mm lens fairly useless on a crop-frame sensor. It ends up being a focal length that is a bit awkward; too short for a lot of portraiture work, yet a bit long to be a walk-about lens. On a full frame camera, yes; good lens. On a 1.5 crop frame it is like using a 75mm lens.
The shortest I tend to go for portraits is around 85 or 90mm on a full frame camera (57mm or 60mm equiv, on a 1.5 crop factor). My f/2 105mm is my go-to lens for portrature and if I want something longer, I pop on my f/2.8 70-300mm lens. I mostly shoot portraits on my full-frame camera, but will use the same lens on my crop frame.
That being said, a 50mm lens is one of the least expensive lenses out there, but unless you find that you shoot at that focal length (and you already have that focal length covered by your 18-55mm lens), the speed and DoF are the only things you will add to what you already own. A lens that sits around and is not used, is no bargain, regardless of the price.