In a message dated 03/07/2002 3:20:31 PM Eastern Standard Time,
hanse112@xxxxxxxxxx writes:
"When I close down to f8, the opening in the diaphragm is clearly a
hexagon about 3mm across. With the bellows cranked out, the front of
the lens is about 27cm, or 270mm, from the film plane, so isn't my
effective f-stop 270/3=90? Or is it? Is this a concern?"
Not exactly my area of expertise, but let me take a shot at this one.
The way I understand it, diffraction is caused when light ways are
interferred with by some sort of an edge or interface, in our example the
edge of the diaghram blades. The rays affected tend to be directed outward,
and fall on the film plane at the wrong location, causing softness.
Diffraction effects increase as a lens is stopped down, because the smaller
the opening the greater the ratio of diffracted vs. not-diffracted waves.
This ration is a function of the absolute size of the opening, not the
f-ratio. So a 200mm lens stopped down to f/32 will have the same amount of
diffraction effects as a 100mm lens stopped down to f/16 or a 50mm lens
stopped down to f/8 (all of which have an absolute aperture of 6.25mm).
Lurking physicists please correct me if I am wrong here.
Paul Schings
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