I am looking at a new lens for my camera (Canon XSi). I have been looking at Vistek, they have a good online search function. Any ways I did a search for EF mount lenses and you can anrrow down your search buy DSLR Lenes Full Frame or DSLR Lenses Non-Full Frame. I am looking for just about any lens, I am not sure what I want to do yet. If I select Non-Full Frame I only get 10 lenses to choose from, If I dont change an thing I get 70. Will It mater if I use a full frame lens on my XSi? I know that a Full Frame lens is based off 35mm, so how much would be cut if I use it on my XSi? Also would my view finder picture be and diferent then the final picture? Here is the link for the search, Find Your Lens DSLR Lenses Full Frame Camera Lenses Canon Zoom Lenses Telephoto Lenses Here are the Lenses I am looking at, Canon EF 100-300mm f/4.5-5.6 USM Telephoto Zoom Lens DSLR Lenses Full Frame 2565A003 - Vistek Canada Product Detail Canon EF 50mm f/2.5 Macro Lens DSLR Lenses Full Frame 2537A003 - Vistek Canada Product Detail Canon EF 35mm f/2.0 Wide Angle Lens DSLR Lenses Full Frame 2507A002 - Vistek Canada Product Detail Tokina AF 10-17mm f/3.5-4.5 ATX DX Canon Fisheye Lens DSLR Lenses Non-Full Frame ATX107DXC - Vistek Canada Product Detail Any advice on if these would be any good or not, limitations, or what you would choose diferent would be greatly acepted.