Search around for info about recent FD-EOS adapters but historically they've been low quality.
Because of the properties of the lenses and lens mounts, an adapter will have to either be a short macro extension tube, which does not change image quality but means it cannot focus to infinity, or a low powered teleconvertor so the lens is a slightly longer focal length. And the second type can reduce image quality. Given the quality of most adapters that have been made, they seriously reduce the image quality.
If you are interested in going to older manual focus lenses you will get better quality if you go for other brands. Nikon, Pentax, Contax/Yashica, etc. Canon FD and Minolta are just not options.