This is just a quick an un-scientific comparison of two Canon lenses.
Both pictures is taken with the same camera, a Canon 20D, but with two different lenses. The large images is 12% of the original size, note the difference in contrast an color!
The small crops i 100% views to show the difference in resolution and contrast.
This picture is taken with the 1.4 extender.
You get what you pay. The 75-300mm is probably OK for use with film, but is not up to the task on a DSLR.