Re: IR Focusing and Diffraction

[Ben Kreunen <ben@xxxxxxxxxxxxxxxxxxxxxxx>]

At 10:46 PM 3/16/00 +0100, you wrote:
>--On den 14 mars 2000, 18:20 +1100 Ben Kreunen
><ben@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> > But basically put, diffraction is made worse by a) smaller apertures and
> > b)  longer wavelengths. Aperture in this case is the physical size of the
> > hole,  not the f number that we are used to (which is a ratio of the
> > focal length  and effective diameter of the hole) so bad diffraction will
> > occur at wider  apertures on short focal length lenses than on telephotos.
> >
>
>No, it is the f-number that matters, i.e. the focal length divided by the
>physical diameter of the aperture. For longer lenses, the physical diameter
>of the aperture at a certain f-number is indeed larger, so that light beams
>are spread by a smaller angle. But the distance from the aperture to the
>film is proportionately larger, and thus the circle of confusion caused by
>diffraction remains the same as long as the f-number is not changed.

Thanks Otto...  I missed that one but it does make sense.

Cheers

Ben

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