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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