ELNikkor wrote:
I'm much more relaxed around rain splatter, fingerprints from curious children nearby, sneeze splatter, excited dog-licking everything in sight, salt-spray, Coke fizz etc etc. when I know it is all hitting my $30 filter rather than the front surface of my $1,000 lens; but that's just me. On very rare occasion, if I believe that my superbly engineered and coated filters may have a slight effect on my final amazing image, I just unscrew the filter, take the photo, then put the filter back on.
I would strongly recommend staying away from a $30 filter. Best of luck.