Large format IR focus adjustment

["Keith Davison" <keith.davison@xxxxxxxxx>]

Is there a rule of thumb to estimate the IR focus shift when using large format 
cameras?  Something like "focus and then move the front standard forward 
by 0.x times the focal length of the lens"?

A couple of my LF lenses are S-Angulon XLs in helical focusing mounts, 
they have calibrated distance scales but no IR focus mark, if I could 
calculate the focus shift for these it would be simple matter to put a red dot 
on the distance scale.

I suppose that just using normal focus and very small apertures is probably 
not a good idea considering the increased diffraction problems at longer 
wavelengths.

Keith Davison

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