What to Do About Scratches on Binocular Lenses: The Right Way to Clean Them
"There's a scratch on the lens. Can it be fixed?" Let's start with the conclusion: scratches on the surface of an optical lens cannot be repaired, and damage to the coating cannot be restored either. There are only two things you can do: clean the lens properly (to prevent more scratches) and assess how much the scratch actually affects the image.
How much a scratch really matters, by degree: a small scratch near the edge (away from the center of the optical axis) is almost invisible in the image. You may notice slight scattering when looking at a light source, but the lens is still usable. A deep scratch in the central area, or a patch of "wipe marks," causes a drop in image quality against backlight and a halo around highlights, which hurts the viewing experience. In that case, sending the unit back to the factory for a lens group replacement is worth considering (weigh the cost against simply buying a new one).
The correct order for routine cleaning (get the order wrong and you are creating scratches yourself): Step one, air blower. Blow away loose dust and grit on the surface. This is the most important step, because going straight to a cloth is like grinding sand into the lens. Step two, lens pen or lens cloth. Use a dedicated microfiber cloth and wipe gently in a spiral from the center outward. Step three, stubborn marks. Dampen the cloth with a small amount of lens cleaning solution and wipe again. Never drip liquid directly onto the lens, since it can seep through the gaps into the body. The lens cloth included with SAGA products is microfiber, so use it rather than a shirttail or a tissue.
Three things to avoid: (1) Wiping the lens with tissues, a shirttail, or toilet paper. The fibers are coarse and hard, and this is the most common source of scratches. (2) Wiping directly with alcohol or other organic solvents. These corrode the coating, which is far more delicate than the glass. (3) "Breathing on it and rubbing hard." Pressing hard drags surface particles across the coating and carves scratches into it.
Everyday protection: put the lens caps back on as you go (in a bumping bag, lenses and keys are close enemies); store the unit in its carry bag when not in use; after a trip to the coast, rinse off the salt spray with clean water first, then clean as described above; in humid southern seasons, air it out and check it regularly.
In one sentence: air blower, lens cloth, cleaning solution, in that order, is the logic of lens care. Scratches cannot be repaired, but 90% of scratches are caused by cleaning.