A foggy Canon 50/1.8 is fairly easy to clean. Most likely the fog is on the surface directly behind the aperture blade.
You can check this visually by looking through the lens from the front at a light source and closing the aperture.
If this is true, buy yourself a lens spanner, and
- take the barrel off the focus mount (by unsccrewing the large ring from the back)
- take the rear optical block off (another span ring)
Then you can clean the dirty surface. Do this with a good cleaning cloth,
micro-fiber, or similar.
Then put back together the other way around. Your risk of course.
Newer Canon 50s can also be disassembled from the front. If you have
a black 50/1.8, there should be a tiny screw in front of the aperture
ring. If you take this out, you can unscrew the entire front block
and get access to the above element surface.
(I've cleaned multiple Canon lenses successfully. They are relatively easy since
the coating is pretty hard).
Fungus you have to treat a little differently. Clean thoroughly, discard the cleaning cloth
and expose the open lens to light for a couple of days).
Hope this helps.
Roland.