focal length - Is an EF 50mm f/1.4 the same as 50mm with an EF-S lens on a Canon 550D?

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?

Answer

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.