: : I find this discusion of filters interesting and helpful. I will reconsider their use. Could someone desribe what the fogging, that may occur with a Tiffen lens, looks like? Does it appear as spots - or is it a coating all over the lens? Thank you.

A generalized fog, though not necessarily perfectly even...
If you have spots (especially with "tendrils"), it is more
likely mold, and it should be cleaned off soon to prevent
etching of the glass.