This is one of those things I wish I can test it myself but I'm not willing to destroy my filter to see how it will change things....
Commonly held wisdom is that less below lens (in enlargers) the better the print image quality. In this wisdom, it is said that below lens multi-grade filters degrade images if not perceptibly - at least theoretically.
There are statements found on internet, some firsthand, some second or third, that in reality, one will be hard pressed to see the difference in actual products even when filters are somewhat scratched. One on this forum has said, because the filter is so thin and close to the lens, it won't affect images.
I have two enlargers that currently, both are fitted with below lens Ilford filters. Personally, I have no seen anything objectionable but since I cannot test the other, I have no idea if it could be better.
Do we have anyone who has actually done tests in this regard? That is, comparison between above and below, then for below, pristine filters and scratched? If so, I would very much like to know what the experiences were.
For this thread, I am not so interested in how it *should be* based on theory alone. I'm more interested in actual accounts.