So yesterday we found out what defects in the lens have no effect on the photos, let's discuss what affects the photos...
1. Fog
Fog is a condition where the optical part of the lens looks like there is fog. If conditions like this can no longer be cleaned, you can only slab it (scrub it off). However, if the optical elements of the lens are rubbed, they can break and the coating layer is lost. It is recommended not to buy lenses with fog, unless you want to take parts from the lens.
2. Mushrooms
Mushrooms usually look like roots, if there are still a few it usually doesn't affect the photo results. You need to put it in a dry box so that mold doesn't grow and spread.
But if there is a lot of mold, you can clean it so that it doesn't form a crust or, worse, fog. Usually when it is cleaned there are traces like halos in certain places which are called cleaning marks. However, if there are no cleaning marks, usually the coating layer on the lens has lifted. Nikon lens types are easy to clean except for the E series type lenses. It is recommended not to buy used Nikon E series lenses that are already moldy.
Source: https://insidecam17.blogspot.com
3. Scratch on the lens optics
Scratches on the optics of the lens, if only a little, will not affect the photo results, but if they are serious they will be visible in your photos. Usually if the scratch is on the front of the lens, either in the middle or the edge, it doesn't matter, but if it is on the optical part of the back of the lens it will affect the results of your photo. It is recommended not to purchase lenses with severe scratches on the rear optical part of the lens.
4. Coating
Coating is a thin, colored layer on the optical elements of a lens which functions to reduce flare and help photograph when backlit.
Even though the coating layer wears off, the photos will still be sharp. Sometimes if we look from the side the color of the coating is uneven and some are missing, this condition is still not a problem and is normal. But it's different for lenses with nano coating, if the coating is lost, it can affect the photo results. So if you want to buy lenses with nano coating, you have to make sure the coating on the lenses is still smooth.