Filter - part of optical path? A requisite?
I note that in some lenses, a filter, even if it a clear one, is required because it is part of the optical path. For example, some very fast long lenses for 35mm, and some wide lenses where the filter is inside the lens.
My question concerns Metrogons and other aerial or mapping lenses that when used in their original mission were (to my knowledge) always used with a filter, usually yellow, sometimes red, and supplied with a clear filter as well. Does their formula require a filter?