Plus, the act of removing and installing a filter may, in and of itself, change the focus somewhat. Perhaps a better test might be to use a film camera, so the entire frame can be used.
I'm pretty much with Godders about using filters. Long ago I stopped using UV and Skylight filters for "protection." In most situations a decent lens hood will protect the lens. When the environment gets pretty crappy, a UV or skylight might be a wise move. The coatings on lenses these days are pretty tough. I was with John Francis once or twice up at Sears Point, and in some situations a filter might have been helpful considering the dust and debris in some of the corners. Shel > [Original Message] > From: Godfrey DiGiorgi > Not adjusting focus isn't sensible as a filter will almost always > shift the best point of focus ... it's two more air-glass surfaces, > regardless of whether it is flat or not, which is going to change the > light path. The key thing with a test is to see whether it has any > other degrading effect on the lens' performance. > > I did these tests 20 years ago and won't spend the time to do them > again. I only use filters when I feel they are necessary ... with a > digital camera, a polarizer and an ND are pretty much all that's > needed, and them relatively infrequently, unless you're doing special > things like IR.
