Well, the jury is still out. It was so hot today that I could see heat waves in zoomed in live view. I will try again later. I suppose I might not have let the lens warm up enough, but the test target is across the street and the pavement was kicking up tidal waves. Anyway, I couldn't tell if any sharpness differences were from the filter, lack of the filter, or heat distortion. I dug through half a dozen Ilford paper boxes looking for anything that resembled a decent quality 49mm UV filter. I found something called a Ghostless UV filter, and one called Cloudy, which appears to be a slight warming filter. Both Pentax brand. They will do for my purposes. bill