On Apr 2, 2006, at 4:24 PM, Cotty wrote:
What sort of test did you have in mind?
I've got the Pentax 77mm and 112mm filters, and one prime and one
zoom
that take those filter sizes. But I probably don't have a tripod
that's
good enough to produce meaningful test results.
Basically, on a tripod with a cable or remote release, lens with
filter,
focus, mid-range aperture (5.6 or 8) shoot. Remove filter, *don't*
adjust focus, shoot. Compare results.
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.
Godfrey
