LIPH From Italy, joined May 2004, 848 posts, RR: 1 Posted (8 years 1 month 5 days 6 hours ago) and read 3800 times:
Looking into the viewfinder of my 30D I can see a little dust spot (shaped like a samll worm) in the lower right angle. Actually it is not a CMOS dust spot because it disappears changing the lens. I tried to look and clean (pretty quickly to be honest) the front and back side of my Canon 100-400 but it's still there. Any idea ? Thanks in advance.
These two comments contradict each other. If the spot is in the lens and you can see it in the viewfinder, then it's going to show up in pictures as well. To confirm this, try several shots of clear, blue sky at different aperatures.
But since you say it doesn't appear in pictures, the spot must be on the mirror. Of course, if the mirror is dirty you would see the spot regardless which lens you mount.
Time to do some testing to figure out which one it is.