Re: pinhole angle of view pinhole vs slit

----- Original Message -----
From: "Murray Leshner" <murrayatuptowngallery@yahoo.com>
>
> What (physics) determines the angle of view for a
> pinhole, notwithstanding the subjectiveness of degree
> of falloff? I suppose coverage is a function of angle
> of view and f.l., but the angle of view is the most
> interesting part, to me.

In the context of pinhole cameras:

There is nothing subjective about the Angle of view, it is purely geometric
and dependant only on the film format (its diagonal) and the focal length.
For rectangular film formats (square included), you basically draw a line
from your pinhole to oposite corners of the film and then measure the angle
between those 2 lines. You can actually draw it to scale on paper and then
measure it with a protractor. Or you can use trigonometry to find it

Angle of view = 2 x ArcTangent [ diagonal / ( 2 x FocalLength) ]

Coverage is another thing, it depends on fall off and the photographer
tolerance to it. Fall off would depend on how far, in degrees, we are
getting away from the center. And if you have to know, in the cosine value
of that angle multiplied by itself 4 times. For instance, at 32.76 degrees
away from the center, the fall off would be 1 stop compared with the center
of the image, 2 stops at 45 degrees, 4 stops at 60 degrees. Very often we
read that a pinhole camera can cover a circle equal to 3.5 times the focal
length, such camera would have about 120 degrees angle of view and the fall
off at the corners of the film would be 4 stops.

> And what about slit cameras?
>
> Does one need a slit the entire length of one axis of
> a given format?, or is there a coverage factor in that
> regard too?

If a pinhole, suitable far enough from the film is able to cover the entire
film, a slit (single slit) would have less trouble doing. You can consider
a slit as a multitude of pinholes placed one after the other. If you draw a
"virtual" circle of coverage with center on each of the pinholes, it is not
difficult to see that the resulting area is the coverage of the slit, it
would basically have the shape of a capsule pill.

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Received on Thu Dec 2 12:12:19 2004