That Canon 50mm f/1.4 that I bought is a vintage lens with the FD mount.
This means if fits perfectly to one of my vintage Canon film cameras, but I can also use it on my Canon DSLR’s with the right kind of adapter, which I found for just a few bucks on eBay.
What I’m going to have to remember, though, is that I will only have manual focus with this lens – which is fine, because I’m not going to use a 50mm f/1.4 for sports, anyway. There may also be a few minor downsides compared to brand new lenses (certain apertures can give you less than tack sharp edges in images) but for $25 I’m willing to live with that.
When you buy vintage lenses it’s good to search online for reviews. It’s also a good idea to buy face-to-face, if possible, so that you can check the condition of the glass. However, if you’re only investing such a small amount of money, it may be worth taking your chances.
Here’s a video by DSLR Video Shooter with 3 excellent examples of vintage lenses for DSLRs.