Re: Pinhole Calculations

Well, actually, the "angle of view" depends on there being a flat film plane
interposed on the hemispheric optimal image distance. The average 'angle of
view' is 1.5FL either side of the axis. Light falloff is a function of the
different distance from the pinhole at different points on the flat film
plane, plus the changing apparent shape of the pinhole as you move off-axis.
Film configurations that approximate the chape of the optimal image,. i.e.
half cylinder cameras, have only the light fall off due to the change in
apparent shape of the pinhole, but no falloff due to change in pinhole-film
distance since theat is the same for the full 180 degrees. They give good
exposure through all 180 degrees.
----- Original Message -----
From: "Richard M. Koolish" <koolish@bbn.com>
To: <pinhole-discussion@pinhole.com>
Sent: Monday, March 11, 2002 9:30 AM
Subject: Re: [pinhole-discussion] Pinhole Calculations

> photoian@earthlink.net writes:
>
> > Query to Larry Bullis:
> >
> >
> > I have experimented with a set of 12 pinholes obtained fromCalumet,
ranging
> > from 0.0059 to
> > 0.032 inches in diameter, on a 4x5 view camera.
> > It quickly became apparent that angle of view is dependent only only on
> > lens to film plane
> > distance. Any of the 12 varying pinhole sizes give the same angle of
view
> > at a given bellows extension.
> > The difference is in the amount of light admitted by the pinhole
aperture.
> > Thus it makes sense that, as
> > the pinhole aperture becomes smaller , admitting less light, the bellows
> > extension must decrease, to
> > maintain the same amount of light, which means the focal length gets
> > smaller. You've given a formula
> > to calculate the optimum pinhole size for a given focal length to give
the
> > "sharpest image."
> > The formula is pinhole(in) = square root FL x 0.0073 or pinhole(mm)=
square
> > root FL x 0.03679.
> >
> > My question is; does this formula really give the sharpest image?
> >
> > First, you've said that depth of field is essentially uniform from
near to
> > far and somewhat soft
> > because of diffraction. Since, for a given focal length,
aperture(pinhole)
> > varies inversly with f-stop, the
> > formula must be designed to balance pinhole against f-stop, one
admitting
> > more light and the other
> > admitting less light. The constant (.oo73 or .03679) is what determines
the
> > answer. So, now the
> > question is; How is the constant determined? Does it give the "sharpest"
> > image or is it just a trade off
> > between exposure time and pinhole size? Do smaller pinholes give more
> > diffraction and thus less sharp
> > images? Using a different constant will gives different answers; what is
> > unique about the given
> > constants?
> >
>
>
> 1. You are right in observing that the angle of view depends only on
the
> distance from pinhole to film, and has nothing to do with the size of
> the pinhole. The easiest way to visualize what is happening is to
make
> a scale drawing of the camera, showing the film plane and the pinhole
> at the desired distance from the film. Straight lines drawn from the
> edge of the film through the pinhole show you the angle of view. Note
> that the intensity of light from the pinhole does fall off as you move
> from the center of the film toward the edge of the film, both because
> the film get farther from the pinhole, and because when viewed from an
> angle, the circular pinhole looks more and more elliptical and has a
> smaller effective area.
>
> 2. The calculation for 'optimal' pinhole usually starts with some
> assumption about the physics of light going through a small hole.
> Since every aperture, hole or lens, produces a diffraction pattern, one
> 'rule' is:
>
> The size of the optimal pinhole for a given focal length is the
> size of the diffraction disk it produces.
>
> The theory behind this rule is that a larger hole would produce a
> larger geometrical beam of light coming through, and therefore a
less
> sharp image, while a smaller hole would produce a larger diffraction
> spot, and therefore also produces a less sharp image.
>
> Note that the size of the diffraction disk depends on the wavelength
> (color) of light. One common assumption is to assume a wavelength of
> 500 or 550 nanometers (nm). This is a green color. If you use
> panchromatic film, red light coming through the pinhole will make a
> slightly larger diffraction spot. But nothing is is very critical
> here.
>
> Nothing in this rule takes into account the f number of the pinhole.
> The f number is just something that results from computing the optimal
> pinhole for a desired focal distance. Because the optimal pinhole size
> increases only as the square root of the focal distance, the f number
> gets larger and larger as the focal distance increases.
>
> 3. Since you have a number of pinholes, you can try some of them at a
> fixed focal distance and what happens. Please report on your
findings.
>
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Received on Mon Mar 11 15:16:00 2002