Re: IR Focusing and Diffraction

[Otto Giesenfeld <otto.giesenfeld@xxxxxxxxxxxx>]

--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.

(I have never seen a good, pedagogical explanation of this fact, but it is
certainly stated in many trustworthy publications, e.g. the textbook "Basic
Photographic Materials and Processes", published by Focal Press.
Presumably, any explanation would become quite complicated as soon as
retrofocus and reversed-telephoto lenses were taken into account, but I
have no doubt that the rule can be shown to be true for such designs as
well.)

Best regards,

Otto Giesenfeld 

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