So I finally got myself a macro lens. A Canon compact-macro lens 50mm. It seems to be working great but I know this lens has been around for awhile. It's an EF lens so it will also fit a 35mm film camera. I was also playing around with an older style EF zoom lens, 75-300mm on my Canon 400D. (not the best zoom lens, I know.) Does anyone know if the image quality is hindered anyway when using these lenses on a DSLR? Should I get rid of them, or what? (Trying to save a few pennies as you can probably tell.)
Thanks.

if the glass is still in good order there is no reason not to use them. they will work fine on your DSLR. the only issues you may come across with older lenses is that the digital capture may not be as sharp as you would have liked (you can compensate for this either in post work or increasing the in camera sharpness settings) and you may also get an increase in chromatic abbrerations with older lenses but thats about it.

its not hindered in my opinion, some of my best macro was done with a 50 year old lens i inherited from my grandfather. sharp as a whistle too! only thing that needed done to it was dust reduction..take a look here(sorry, forgot html!)

http://www.caedes.net/Zephir.cgi?lib=Caedes::Infopage&image=gse1978-1166985556.jpg

works well for macro, but when i tried my longer lenses, they didnts seem so good, too much aberation and fringing, and a lot of softness, however, im talking OLD lenses.

Remember any "EF" canon lens should be fully compatible with any EOS series camera. Since you are using the 400D you will not et the full-frame picture-which you probly already know. The only thing is that your glass determines your image quality about as much as your camera, so get the best you can afford. The 50 you have is a pretty good lens, though it is only a 1:2 macro. If the 75-300 you have is the one I think it is then its not too bad for a budget tele. I actually sold mine some time ago. If I get another long lens it will more than likely be a prime.
My 2¢