Even worse than lost/stolen is damaged, IMO, just because it introduces a much bigger gray area. Let's say I lend someone my Canon 100-400mm, it comes back loo

Even worse than lost/stolen is damaged, IMO, just because it introduces a much bigger gray area. Let's say I lend someone my Canon 100-400mm, it comes back looking mostly okay but with a relatively small scratch on the front lens element that I'm sure wasn't there previously. Still works mostly fine, but now when I shoot with the sun in the frame, I get lens flare that didn't previously exhibit. How do I prove the renter subtly damaged the lens, if it comes down to my word against theirs? I would need a lot of iron clad assurances that I'm covered in return for renting out my various $1k+ lenses for a few tens of dollars. Though ultimately, I don't currently live in a covered city (I'm in San Diego) so this is theoretical in my case.