At 18:58 3/9/02, Dean Hansen wrote:
Hello OM'ers,
Frieder Faig and Chris Barrett both nailed it with the question of
aperture--I was stopping down (to f8) too much. The reasons why this
gives lower resolution will take some study.
Frieder and Chris hit on it, and with something I wouldn't have suspected
at f/8 although it's obviously influenced by magnification. I believe
Frieder's original posting mentioned diffraction. In general, as a lens is
stopped down from wide open toward narrowest aperture, its resolving power
improves until an optimal aperture is reached. After that, stopping it
down further degrades its resolving power.
From wide aperture to the optimal, resolution improves because certain
aberrations are reduced by eliminating ray paths at the edges of the lens
elements and using those nearer the lens axis. As the lens stops down
further from optimal, aperture diffraction starts to degrade
resolution. As light passes through a circular (or nearly circular) hole,
its ray paths spread out and the effect is called diffraction. The lens
diaphragm is a circular (or nearly circular) hole. As it is made smaller
in diameter, the effect of diffraction increases. What Frieder alluded to
was the spreading of light caused by aperture diffraction becoming so great
that it exceeded the maximum diameter "circle of confusion" (CoC) which
defines front and rear depth of field (DoF) boundaries.
What's CoC? At critical focus distance a point is a point. At some other
distance, it's a spot called the CoC. However, but a human can't tell it's
a spot unless its big enough. That's the maximum diameter CoC. This means
the DoF boundaries around critical focus distance are how far out of focus
points become that they are detected as "spots" and things no longer look
"in focus." If the light spreading caused by aperture diffraction becomes
greater than the maximum diameter CoC, *nothing* will appear to be in focus.
There are two excellent discussions about optics on photo.net:
The Lens FAQ, an overview without techno-babble:
http://www.photo.net/learn/optics/lensFAQ
[See Q8, Q21 and Q22]
The Lens Tutorial with all the techno-babble:
http://www.photo.net/learn/optics/lensTutorial
[See two sections: Circle of Confusion, and Diffraction]
I never would have thought this would occur at f/8, but then I've never
done anything beyond 1:1 magnification!
-- John
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