I have an FD adapter for an EOS 100D, a 50mm and a 210 Sun Optics zoom lens. They both produce a type of gloss/glare/fuzzy image after around 10 minutes of being attached, is this normal? Is this humidity from these old lenses? I never leave these old lenses on the camera when not in use because I've heard this could damage the camera. Is there a way to rectify this? Like a de-humidifier? Or is this a lost cause? The image shows a bit of this effect of blur. Canon eos100d +sun opt.210mm zoom +2x adapter=420mm.
The softness is probably from the 2x teleconverter and the glass in the FD->EOS adapter. That's actually like stacking two teleconverters, since most of these adapters that use a glass element to help you achieve focus to infinity are simply short teleconverters.
Teleconverters nearly always add softness, and while they do increase the focal length, they also reduce the maximum aperture, so everything gets darker, and your camera may have some serious trouble focusing correctly because it doesn't have enough light hitting the AF sensor array to actually detect phase or contrast well enough to find focus. The 100D's upper limit for autofocusing is a max. aperture of f/5.6. At f/8, it won't be able to latch onto focus.
Given that you're using an f/4(?) lens to begin with, throwing on a 2x tc brings you down to f/8. And with the FD->EOS adapter, you're liable closer to f/11 as your maximum aperture.
My recommendation would be to take off the 2x teleconverter. That's liable to remove most of the softness and focusing issues you're running into. Using a teleconverter is a far far cry from using a supertelephoto lens, which is why I stopped using a 1.4x tc on my EF 75-300 III, and ponied up the $1000+ for an EF 400mm f/5.6L USM instead.
Sometimes, there are no shortcuts.
The most likely culprit for fuzzy images when using this lens combination is the glass you will find in your FD-EOS adapter. The low quality glass that is often used greatly degrades and softens the image.
If you are using a glassless adapter then the fuzziness could be caused by the lack of infinity focus. This is due to the differences in old FD and new EOS mount flange distances.
Also please bear in mind that old film lenses, due to their age are likely to deteriorate image quality if not looked after properly.
Film Lenses are also not specifically optimised for digital cameras e.g. No digital coatings. So you might get some unusual results.
I personally have a glass FD-EOS adapter. I stopped using it because of similar issues. If you would like to use old Film lenses on your camera you would be better off trying an old Nikon F of M42 mount lens plus adapter.