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Old 30-01-2013, 12:27 PM
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Wavytone is offline
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
Interference filters are designed on the assumption the light passes through perpendicular to the filter. The filter consists of two thin semi-transparent, semi-reflective layers that are separated by an exact half wavelength, or multiples of this.

The reason they don't work well with fast lenses is that the light entering the filter at an oblique angle travels a longer path inside the filter and hence it will transmit light of a longer wavelength than it was designed for. You can see the same thing if you take say an OIII filter, hold it up and watch what happens as you tilt it.

f/3 is about the practical limit.
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