Tamron makes a 75-300mm /4.0-5.6 LD . For Canon it's $149.00

It is quite a strange situation, but usually one of the sharpest lenses that any camera company makes is the normal 50mm F1.7 lens. Canon makes a lens in the 50mm Focal length with a F1.7 aperture. These lenses are very highly corrected to deal with abberations, and at the same time produce some of the finest image quality that any lens can produce. Even though, I use a 50mm F1.8 Nikkor lens, if you buy a Canon F1.7 lens and stop that lens down to F4 or to a smaller aperture you will end up with images with stunning image quality and sharpness. And the best part of all you will probably end up spending less then $80.00 for the lens. Good Luck in deciding what lens to spend your money on. Astronomer, Syracuse, NY.