Originally Posted by c6h6o3
Originally Posted by MikeK
lens that stops down to f128..while you get great DOF the results look a little better that a pinhole.
Mike
I've gone down to f128 with my 19" Artar and the results look a LOT better than a pinhole. Sometimes movements just won't help.
I've finally researchd "diffraction" using a very informative book from the past courses in physical optics: "What is Light", by A.C.S. van Heel and C.H.F. Velzel (translated from the Dutch by J.L.J. Roenfeld).

I was trying to condense "diffraction effect" inot a bite size formula ... and I've had no such luck. Diffiration is the result of light waves "bending" around the edge of an opaque substance. It might help to note that *no* substance is perfectly opaque - all will be penetrated by, and refract light, to *some* extent.

"Diffraction" as we know it, is a result of scattered light waves from an edge (one side of a camera diaphram) interfering with other similar scattered waves from the surface opposite. The effect is a result of the *phyiscal size* (actual size - not the relative size indicated by the term f/stop) and the wavelength of the light in question. This is a wavefront propagation and the total effect will be modified by distance from the diaphram.

An "Aperture" of f/128, given a long focal length, say 19", is much larger in physical diameter than a f/128 aperture of a shorter focal length lens - say 10" - therefore diffraction errors are much less in the "long" system.

As for the optical system itself - it is not entirely accurate to say the the lens has "*no* effect on diffraction - it does, but much less of an effect than the aperture diamter - wavelength - propagation length - combination.

I had hoped for a simple formula - hah! The description and mathematics pertaining to diffraction are lengthy - and *invloved*, consuming many pages of the book.

The only subject more complicated is "Polarization" - after intense study - and passing grades - I *still"* do not wholly understand what the hell is going on there.