Re: IR Focusing and Diffraction

[Ben Kreunen <ben@xxxxxxxxxxxxxxxxxxxxxxx>]

>
>Could someone explain the problem with diffraction at small apertures?
>This says to stop down, but not too far. What is too far? If a view
>camera lens stops to f45, is it OK to go to f32? I was thinking of
>trying some IR pinhole photos at about f128. What can I expect? I've
>heard of the "circle of confusion", but this is confusing.

Try: <http://micro.magnet.fsu.edu/primer/java/diffraction/index.html>

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.

Don't know which aperture...  then test, test, test as with anything 
else.  A simple one for diffraction and general lens sharpness is to 
photograph a test chart, or, if you don't have one, a piece of paper with 
some fine black lines on them. Use a tripod to avoid focusing differences. 
Keep the overall exposure constant and photograph it at each aperture of 
your lens. Mount the neg into a 35mm slide mount and project it really BIG 
as possible on a wall and compare the sharpness of each aperture. You can 
decide for yourself which is your sharpest aperture for each lens, and at 
which apertures your images really fall apart. If you don't have a 
projector, enlarge it as big as you can and print a small section from the 
middle and corner of the image.

Your home made test target should at least contain crossed lines, both 
diagonal and perpendicular, placed at the center and corners of the frame 
so that you can evaluate the effects of other distortions as well.  e.g.

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If you're using a zoom lens, do the test for each end of the zoom range and 
maybe one in the middle.

It doesn't have to be done on IR film, in fact the finer the grain, the 
better. Techpan will give you results that will really test your lens' 
resolution (and can make you very disappointed with 35mm SLR lenses) If you 
wanted to extrapolate your results to IR then allow 1 stop wider than you 
find with this test.

Can't be bothered with all that....  then find a flat subject, say a brick 
wall with fine cracksand harsh lighting and photograph it at different 
apertures. We're not after pretty here, just flat with fine detail.

Cheers

Ben

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