I have a Canon 550D with the standard 18-55mm kit lens. The 550D has a 1.6× crop factor.
My question is very simple but whenever I've asked before (Yahoo Answers) I always got overly-complicated answers which confused me even more. So here goes.
If I get the EF 50mm f/1.4 USM lens, will it be the same as setting my EF-S zoom lens to 50mm when attached to the 550D? Or will it be the equivalent of an 80mm EF-S lens?
I'm going to go with the very simple answer here:
The lens does not change. The f/1.4 50mm EF lens has exactly the same focal length (and everything that goes with that) as your EF-S zoom lens set to 50mm.
The "crop factor" is not useful for comparing different lenses on the same camera. It's only useful for comparing lenses across different camera formats — which means different film or sensor size.
And then, because I can't help myself, I'm going to go a bit further. This seems like overcomplicated nonsense, but it's actually both pretty simple (it only seems complicated — honest!), and a reasonably useful tool to know about.
So, in case you want to chase things further, once you're definitely unconfused and clear about the basics, see:
Which give some depth and background. But still, remember, the answer to your question is simple: focal length is a property of the lens, and you can't change it without more optics.
As the previous answers stated correctly, the Field of View (FoV) will be the same with the two lenses. That means that you will capture the same scene with the two. However, note that there is a difference in the maximum aperture between the lenses. The 50/1.4 will let you shoot in lower light, and can produce shallower Depth of Field (DoF) than the kit lens. It is also much sturdier than the kit lens.
I added this answer b/c your question was "...will it be the same as...". So, to be picky, no - it will not be the same, but it will be pretty similar.
The field of view (the picture you see through your lens) will be the same with any 50mm lens - regardless if its EF or EF-S.
Both lenses are 50mm. the only real difference is that the EF-S can be a smaller piece of glass, because it need only illuminate a smaller sensor. The EF must be able to illuminate at full frame sensor, so it is a physically bigger piece of glass. This is the primary reason that EF-S lenses are less expensive.
It will be the same as your zoom set to 50mm.
A 50mm focal length lens is a 50mm focal length lens, neither lens "knows" what sort of sensor you have behind it. It's just that the smaller sensor captures less of the projected image, so the resulting image is cropped. This is similar (but not the same) as what you'd get on an FX sensor with about an 80mm lens. This is true whether you are using an 18-55mm zoom at 50mm, or any fixed focal length 50mm.
