If a lens is not apochromatically corrected, especially an older lens that may not even be an achromat, does using a colored filter (typical yellow#8, red#25, etc.) noticeably increase sharpness? It makes sense to me that it would eliminate chromatic abberation, but how much does that affect sharpness in real world B&W photography? Would it have to be a really dark filter that only lets a narrow range of wavelengths through?
I have used minus violet filters for refractor telescopes to cut down on CA with bright pinpoint objects (stars, etc). This has helped somewhat. Not sure about yellow though. I haven't tried that.
The reason a filter will, to some extent, help sharpen an older lens is that not all colors focus at the same point. Achromats are corrected for two of the colors. APO lenses are corrected for all colors. The filter helps by blocking some of the color that is out of focus, ie - red. If the color is no longer there or is reduced, it will make the others appear sharper. Too techie for me but that's about as good a I can describe it.
A narrow-pass filter (such as those used for RGB tri-color) will usually improve the image very slightly, but with a very well achromatized or truly apochromatic lens, may do so less than the filter's two additional surfaces degrade the image. With a very simple lens like a meniscus or periskop (a double meniscus, like a Rapid Rectilinear with singlets instead of doublets front and rear), the elimination of chromatic aberration may be significant, but a filter won't help spheric aberration, and there are few lenses with a lot of one that don't also have visible amounts of the other. Add the increased exposure required due to the filter, with increased chance of subject or camera movement and issues of reciprocity failure, and IMO it's usually better to accept the limitations of lenses and work with them, rather than against them.
I haven't seen much to complain about with my pre-War Tessars, filter or no filter...
If a contact print at arm's length is too small to see, you need a bigger camera. :D
Several years ago one of the photo magazines (probably Phototechniques) published a high school student's science project.
He had taken a Canon 50 mm prime lens of fairly current design and took compairison photographs of a shopping center parking lot with
B&W film. With and without a "band pass" filter he purchased from an Edmonds Scientific catalogue. (A band pass filter lets a very narrow band of visiable light through.)
Even after printing reproduction in the magazine the differences were startling.
As I remember the differences were at least equvilant to 1/2 the enlargement ratio.
Yes, a strong monochromatic filter like a K2 (medium yellow) usually improves sharpness, particularly with older lenses or if you're using one cell of a convertible. Focus with the filter in place.
Thanks for the info. I have a rapid rectilinear that I want to use one cell of, and needs all the help it can get!
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