08-22-2015 07:27 PM
Solved! Go to Solution.
08-22-2015 09:19 PM - edited 08-22-2015 09:22 PM
They are most likely dust particles on the bottom or top of the focusing screen. Since you said they don't affect picture quality it is up to you if you want to spend $ to remove a nuisance that's not doing any harm. You are probably looking at a signifcant fraction of the camera's value if you have to send it in.
You can try blowing out with a squeeze blower (DO NOT USE COMPRESSED AIR CAN) but be careful you don't hit anything with the nozzle. You might be able to remove the specs from the focus screen.
08-24-2015 10:58 AM
Using compressed air to blow out the interior of the camera can blow dust into the area above the focusing screen. The focusing screen is not "sealed" -- so while dust and debris probably wouldn't wander up there on it's own, a good blast of air can send things flying everywhere.
It is possible to remove the screen (there are 3rd party companies that make optional focusing screens with different types of focusing aids, gridlines, etc. etched into them.) and they provide instrcutions on removal and replacement.
But this is not normally the sort of thing you'd want to do to your camera and you risk damaging the focusing screen. They are very easy to damage and normally should not be cleaned (they should never be touched because if you get skin oils on the frosted surface you'll probably damage the screen trying to clean it off.)
People who work on them normally wear gloves (latex "surgical" type gloves) to avoid getting any skin oils on the screen, and they protect the reflex mirror (to avoid scratching while removing or inserting a focusing screen.)
I probably would just put up with the dust and not bother to try to clean it out.
08-24-2015 12:12 PM
It is likely dust resting on the "top" of the focusing screen. The screen is a frosted surface (usually the top is frosted and the bottom is glossy). The camera is projecting a focused image onto this screen because the distance from the lens, to the imaging sensor inside the camera is exactly the same distance as the distance from the lens, to the mirror, to the focusing screen.
When you look through the viewfinder, you are actually looking at an image being projected onto the focusing screen (sort of like a rear-projection TV screen). This would sort of be like having dirt on the "inside" of the panel of a rear-projection TV -- it's not on the surface you can touch and would require removing the focusing screen to blow the dust off the back of it.
When you take a photo, the reflex mirror swings clear and the light path goes directly from the back of the lens to the imaging sensor without ever going through the focusing screen -- hence it will not affect your images at all (you could have rocks up in that chamber and it wouldn't affect the images.)
My 5D II camera has user-swappable focusing screens (Canon sells optional screens) and they include a special pair of tweezers with a hooked end to make it easy to open the focusing screen compartment and swap the screens without actually having to "touch" the screen with your fingers. But your camera isn't designed to have swappable focusing screens (although third parties do make them.)
While it is possible to remove the screen and clean it, it must be done with great care because it is easy to damage that screen (especially if you've never done it before and aren't sure how to remove and re-insert the screen.) Since the dust wont show up in an image, I would leave it alone. If you choose to attempt clean it anyway, it's your camera and your choice to accept the risk. But it could lead to a more costly repair if something goes wrong.