You are thinking about buying a Canon EF 200mm f2.8L II aunto focus telephoto lens and am wondering if it will fit on both your canon rebel k2 35mm and also your digital canon rebel XSi?

Short answer,"yes." The Canon EF 200mmm lens is designed to fit a "full-frame" Canon EOS camera - film or digital. It will also fit all Canon DSLR's that have the smaller APS-size sensor, including all of the Digital Rebels.