Re: pinhole photography/ calculating focal length

Bozhidar Dimitrov (
7 Jul 1997 13:29:51 GMT

Geoffrey V. Brown ( wrote:
: >Well, I've got some extra film and want to mess around with some stuff
: >this weekend.
: >I'm thinking about trying some pinhole photos, but doing them with my
: >canon T-90 with the lens removed and a plate in its place. I'm
: >wondering how if anyone has any suggestions on how to calculate the
: >focal length.

The following document contains some information on pinhole cameras. I
think you will find it useful, but it still doe nots discuss focal length
of a pinhole camera.

The notion of a focal lenght might not be applicable to a pinhole. Consider
the following few points:

[from the photographic lense FAQ] Q1: What is focal length? A: A 50 mm
lens produces an image of a distant object on the film that is the same
size as a pinhole 50 mm from the film.

A pinhole has an angle of coverage of 180 degree coverage, and this
is independent of the distance between the hole and the screen (note that
you need an infinitely large screen to capture the image).

Varying the distance from the hole to the film changed the size of the
objects (as well as their brightness, but NOT the relative size of objects
at different distances.

Hope this helps,

     _\\|//_        Imagination is more important than knowledge...
   0(` O-O ')0                                              A. Einstein
 BOZHIDAR D. DIMITROV               DESY, Notkestrasse 85, 22607 Hamburg, Germany
~~ <__>   <__> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~