Pierre Vandevennne wrote:
I was browsing Astronomik's site, learning about filters and found many
comments such as this one

"The optimal focal ratio for the use of this filter is between 1:3.5 and
1:6. The range of application is from 1:2.8 up to 1:15"

How can a filter be better suited for one focal ration than for another?

If the filter is primarily for imaging/photography, the filter may be
too dim to be used with slow lenses/mirrors. In visual use, the apparent
surface brightness of an object is governed by the aperture and the
magnification; in photography, it is governed by the focal ratio.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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