In a couple days, I may buy a Tamron 100-300 lens used for $50. The thing is, I only use KM lenses on my 5D, and have read that other companies can get stuck. Does anyone have any knowledge of this happening? I'm afraid to try a third party, so any help would be greatly appreciated.

Thanks.

I don't have a digital but I use Sigmas on my Maxxum 70 and have never had any problems with any one of them getting stuck. I know its not the smae camera but it the same mount. I say go for it that 300 becomes 450 on your 5D; 50 bucks for a 450mm lens isn't bad.

I have never heard of a Tamron lens causing a problem with a KM dSLR. Different story with Sigmas, but Tamrons and Tokinas seem fine.

I have a 7D and I have 1 Km and 4 Sigma lenses, never had a problem.