Keep your camera clean and intact.
Do the eyepiece first: A brush, microfiber cloth, or cotton swab is good for the center. You may need lens cleaner to degunk corners.
Now for the viewing screen: Turn camera upside down and give it a blast with an ear syringe or blower brush. Dirt still there? Try a cotton swab (see text).
And now, gulp, the mirror: Start with a brush. Microfiber cloth or cotton swab with lens-cleaning fluid can be used to get rid of fingerprints and the like. But be careful! The mirror is very delicate.
Unfortunately, dust and other dirt gremlins also accumulate inside the finder system, often when you change lenses, focusing screens, or prisms. A tiny piece of gunk can land on the mirror and then, when the mirror snaps upward, the gunk is transferred to the viewing screen and beyond. Probably, when you're taking pictures, you can ignore a blob, thread, or whatnot in the finder system. But at other times, when you're looking through the finder the glop is downright annoying. And perish the thought that you have dirt or, worse, a fingerprint on the mirror!
What's the best and safest way to clean inside a finder system? Let a professional repair shop do the work. But if you are handy and feel you must do the cleaning yourself, we'll tell you how we do it. Warning! First, an overenthusiastic cleaning or a heavy hand can put more junk into the finder system. And second, mirrors, all of which are front-coated, are very delicate indeed.
Let's start real easy, with the finder eyepiece, and see if that clears up the problem. Light dust or dirt is always best removed by a good-quality, clean paintbrush or a lens-cleaning brush. A microfiber cloth will usually lift off central fingerprints nicely. To clean the finder corners and remove any muck, including fingerprints, stuck to the glass, use a cotton swab moistened with lens cleaner or denatured alcohol (not rubbing alcohol).
When you look through your viewfinder after cleaning, do you still see dirt or other particles that don't belong? They're probably on the underside of the viewing screen. If your camera has a removable focusing screen, now's the time to remove and clean it with a microfiber cloth, as you would a lens—but lay off any lens cleaner (as we'll soon explain). No changeable screen? Remove the lens, turn your camera body upside down (so any dislodged dust or dirt won't fall back into the camera), and give the underside of the focusing screen some blasts from an ear syringe or blower brush.
Dirt still there? You'll have to go after it with a cotton swab. But keep it dry. Alcohol or even lens cleaner can streak the bottom of the plastic focusing screen. If you look through the finder at the same time you wield the cotton swab, you can see just where the swab head is and pinpoint the position of the dust or dirt. But be careful not to get the swab near the mirror!
Dirt still there? Try again. And don't be surprised if the cotton swab leaves cotton fibers. A blast of air will usually get rid of them.
OK, it's mirror time. A swipe with a brush will usually remove most particles. If there are tenacious bits sticking to the mirror and they don't affect the view, we advise you to leave them there. If it's a prominent fingerprint or the like, gently and oh-so lightly rub with a microfiber cloth or a swab moistened with lens-cleaning fluid or alcohol. Do not apply pressure on the mirror or you may damage it or throw it out of alignment.
We won't wish you good luck with your cleaning. Luck may play a small part, but care is far more important!
And when in doubt about cleaning, don't do it!
