We're talking about issues with lenses here...
Factors that will not affect picture quality (considering cosmetics, not working condition):
- small- medium amounts of dust inside the lens (this is extremely common and nothing to worry about)
- light fungus (while light amounts of fungus won't affect quality, if it's not taken care of the thing about fungus is that it keeps growing)
- light scratches on the glass
- external wear of the lens body
- minor coating damage (lenses have a protective coating on the glass. Sometimes it can be damaged or etched by water marks that aren't cleaned off, salt, or other substances that haven't been removed in a timely manner)
Factors that will affect picture quality:
- Super heavy dust (will create a haze or soft focus effect)
- Heavy fungus (also creates a haze or soft focus effect)
- Tons of scratches, deep scratches, chips, cracks in the glass (if you're shooting with a large depth of field, these may actually show up on your images as spots, lines, etc.)
- lens separation
- glass elements that have fallen out of place/off track (this will prevent being able to focus properly, but is rare)
- major coating damage, etched glass (this usually occurs if there is extreme fungus, or if the lens hasn't been taken care of properly. Example: Soda was spilled on it and not cleaned off well)
Ways to prevent these things include first and foremost taking proper care of your equipment! Keep caps on your lenses at all times when not in use, use protective filters, keep stored properly, keep away from food, liquids, dirty environments, extreme weather conditions, and humidity. If some of these issues have already started, a professional cleaning may be a good idea. For more info. on how to keep your lenses clean, and how to clean, refer to our previous posts: Camera Killers- Dust & Fungus, and Keeping Things Clean.