P.J.:
What Paul is talking about is diffraction due to the wave nature of light. A
larger aperture has more resolution.
Light tends to "bend" around corners (like the diaphragm blades). When a
parallel beam of light enters a lens, the rays spread out evenly, making it
impossible to focus sharply, even with a hypothetically perfect lens. The
amount of diffraction is proportional to the wavelength of the light divided by
the diameter of the aperture. The resolution of the silver oxide grains on the
film is up to about 6000 dpi on the best film (i.e. Kodak Technical Pan). If a
small lens aperture yields resolution significantly worse than the film grain,
your pictures will look blurry. Loss of resolution is yet another tradeoff when
selecting aperture.
I know this stuff because I'm an amateur astronomer. I have a 10" diameter
reflecting telescope, which has a resolution that is usually better than what
the atmosphere allows (AKA "seeing") when looking at stars. A common astronomy
technique is to stop down a large telescope on turbulent nights, because too
much resolution just emphasizes how the seeing is distorting the image.
Matt BenDaniel
------------------------------
Date: Tue, 21 Mar 2000 23:28:17 -0600
From: "Paul D. Farrar" <farrar@xxxxxxxxxxxx>
Subject: Re: [OM] lens contrast, DOF limitations or my brain?
Diffraction.
Paul
At 10:17 PM 3/21/00 EST, you wrote:
>Thanks to Gary R. and John Hudson for pointing me to the URLs for 50mm lens
>comparisons. Funny thing is that I have looked at Gary's tests dozens of
>times but never taken the time to browse the 50mm lenses. Oops!
>
>Okay, so here is my new question:
>I'm getting into macro work and have noticed a somewhat annoying phenomena.
>If I maximize depth of field (or at least preview to get the subject in
>focus), the subject is in focus but definitely muddled. With a shallow DOF,
>the subject is out of focus, usually, but the parts that are in focus jump
>out at me.
>I have three thoughts on the source of this:
>1) a loss in contrast of a particular lens at smaller apertures
>2) some sort of optical trade-off of a larger DOF in all lenses
>3) or just my own eyes distracted by other items in the frame becoming more
>in focus
>Any thoughts on the cause and solution to get the subject in focus and
>preserve 3D effect? I am trying to solve the problem by using lenses at
>their optimum aperture and also trying to set the depth of field for just the
>subject. Two tasks which are not exactly complementary in macro work.
>Anybody else have/had this problem? What are/were your remedies?
>
>--p.j.
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