Does anyone use UV filters on their lenses? I've always used one mainly to protect my lens, but it seems a bit pointless putting a clear glass filter over an expensive lens. Isn't it just an extra layer of glass for the light to travel through, or doesn't it make any difference? I'd be interested to hear your opinions. Also, I suspect that those cheap £5 - £10 UV filters from ebay may only be plain clear glass. Or am I wrong?