Cleaning an SLR mirror requires special care—especially in older SLR's. The reflex mirror has a silver coating on its front surface. The silver coating reflects the light from the lens to the focusing screen. Since the silver coating is very delicate, rubbing the mirror with a dry tissue can cause scratches
An ordinary household mirror has the silver coating on the back surface of the glass. The glass then protects the silver coating. The front-silvered mirror, although more delicate, eliminates secondary reflections—reflections from the glass surfaces as well as from the silver coating. Manufacturers later added a protective coating over the silver. The protective coating reduces the chance of damage from cleaning the mirror.
If possible, try to restrict your cleaning to blowing off the mirror with a hand blower. The mirror, sitting inside the camera, isn’t as likely to have fingerprints. If you do have to apply lens-cleaning solution, you can use the lens tissue or cotton swab. The soft Pec*Pad, however, may be the safest wipe to use. Using very little pressure, work in horizontal strokes to apply the solution..
As long as the swab or cloth is wet with the solution, there’s not much of a chance of damage. But when you dry the mirror, the danger increases. Use a new Pec*Pad or lens tissue. If you use a microweave lens cloth, make sure it's clean. Work in gentle, horizontal strokes to dry the mirror—be especially gentle with the older SLR’s. Check your work by fogging the mirror with your breath; the fog should appear uniform, indicating that there are no streaks or smudges on the mirror. Wipe off the fog using the same horizontal strokes.
Here again Eclipse™ offers a special advantage. Using Eclipse™, you can just wipe off the fingerprints or other smudges—you don't have to clean the entire mirror. And, because Eclipse dries so quickly, you don't have to wipe the mirror with a dry tissue. As mentioned earlier, it's that dry tissue—not the wet tissue—that normally causes the damage.