Re: Angle of ligh

Bill Erickson writes:
>
> Since you brought this up, there are two factors influencing the intensity
> of light at the film plane, the distance from the pinhole and the angle off
> axis. As you move off axis of a flat film plane, the distance from the
> pinhole to the film grows, and the apparent shape of the pinhole changes
> from round to narrower and narrower. The so called fourth power of the
> cosine law governs. The intensity at any point on a flat film plane equals
> the intensity at the axis point times the cosine, to the fourth power, of
> the angle off axis. When you curve the film around the pinhole you
> counteract half of it because the film is always the same distance, and the
> only darkening you get at the edges is due to the change in the apparent
> shape of the pinhole.

    That's true for half-cylinder cameras. For oatmeal box or paint can
    cameras, the film gets closer to the pinhole as it goes around the
    cylinder (at least in one dimension), so there is at least some
    correction for the pinhole shape factor also.
Received on Tue Feb 12 17:02:02 2002