At 10:49 AM +0800 7/20/01, Clemente Colayco wrote:
Gee where do you guys learn all these?? Or how do you manage to find the
time to delve into such esoterica.
Well, a bunch of this came up about a year or so ago when we were
discussing resolution limits for digital cameras, and back then I
just got my high school physics text (halliday&resnick, which will
date me precisely for some of you) and looked in the index under
diffraction. Since we moved last fall the book is still in one of
the dozen-odd boxes holding up the guest mattress, so I faked it.
paul
----- Original Message -----
From: John A. Lind <jlind@xxxxxxxxxxx>
Subject: Re: [OM] Aperture Modification
At 22:49 7/19/01, Paul Wallich wrote:
>At 11:00 PM +0100 7/19/01, Chris Charlton wrote:
>
>>Just a question, after reading about Ansel Adams,
>>"F64" club using very small apertures to gain the
>>maximum depth of field and detail, does anyone think
>>it would be possible or worthwhile modifying a cheap
>>wide angle (eg Vivitar 28mm) to go down smaller than
>>F22? Would there be any noticable increase in depth of
>>field or detail captured?
>
>You would get more depth of field, but you would start
>losing sharpness dues to diffraction. At f/64 a 28mm
>lens would have an aperture of about 0.44mm. The general
>formula for diffraction is that the first minimum (roughly
>the width of the central spot) is at an angle of L/A, where
>L is the wavelength of the light and A is the aperture. For
>yellow light, L is 600 nanometers, and A is about 440,000
>nanometers, so L/A = 1.36 milliradian. That means the smallest
>spot you can image is in the neighborhood of 28 mm (the focal
>length, i.e. the approximate distance the light travels from
>the aperture) x 0.00136 x 2 (cuz the light spreads out on all
>sides). That would give you a spot size of about 0.075 millimeter,
>or a resolution of about 15 lpm. (I'm leaving out a bunch of
>stuff here, I know, but it's close enough).
>
>So the answer is, you could do it, but it probably wouldn't be a
>great idea.
>
>paul
>
>--
>Paul Wallich pw@xxxxxxxxx
A spot size of ~0.075mm is about 3x the circle of confusion maximum
diameter (0.025mm) for 35mm small format. What's that mean? The max.
dia.
CofC is as large as a spot can be on the *film* before it becomes
discernable as a finite spot and not perceived as an infinitessimally
small
point, by an average unaided human eye, at normal viewing distance, in a
*print*. In other words, photos made using f/64 with a run-of-the-mill
28mm prime lens would be very, very soft (zero sharpness anywhere). This
is the same reason pinhole photographs look severely soft-focused; the
diffraction limiting Paul spoke of in his posting.
-- John
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Paul Wallich pw@xxxxxxxxx
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