I loaned my 300 f4 IS to a girl at a soccer game (a friend, but I was working on more) I mentioned that it was a prime lens. After about an hour she came over with a dreadful look and told me she was really sorry, she'd broken it. How? I said. "It won't zoom anymore" she explained. I explained the prime thing again and she admitted she had no idea what that meant or that there were any lenses that would not zoom. She swore it was zooming when I first gave it to her.