> I would like to make a VERY basic camera obscura. To show elementary students how the
> eye/camera works. I was hoping to make one out of a black painted shoe box, hole on one
> side, tissue paper on the other.
Here are some of my opinions about how to make a camara obscura:
Shoebox is fine for the "body". I'd suggest that instead of tissue paper you use frosted plastic material available at
most art supply stores - it is very similar to ground glass and more stiff than tissue paper.
You can probably get away with using a magnifying glass for a lens ... if you stop it down to a small aperture - or
maybe reading glasses sold at a department store, VOA or Salvation Army ... strength should be 4 diopters which will
give you a focal length of 10 inches roughly.
A 3 diopter lens will be about 15 inches and a 2 diopter one 20 inches. Chose one depending o how deep your shoebox is.
You can combine reading glasses for doubling the diopter strength so you could get a 4+2 = 6 diopter and that would be
about 6 inches or so in focal length. Anyway, 40 inches divided by the strength in diopters gives you the approximate
focal length of one or a combination of reading glasses.
To find the approximate focal length of a reading glass make an image of sun on a piece of paper and measure the
distance from the lens to the paper in inches. That is the approximate focal length. To find the strength in diopters
divide that focal length into 40. If you make the measurement in cm then divide the fl in cm into 100.
Once you know the focal length that will give you an idea of how "deep" the camera body needs to be. If you make a
"nesting" body then you can make small adjustments to the distance between the end to which the lens(es) are attached
and the ground glass plastic on the other end to bring various subject distances to a focus. It is probably useful to
build a "shade" over the frosted paper end to try to minimize extraneous light from washing out the image formed by the
lens.
Stopping the reading glass down like by covering with a piece of black tape into which a hole is punched with a paper
hole puncher should improve optical performance. If the image becomes too dim then enlarge the hole. The diameter of the
hole divided into the focal length gives you an idea of the effective aperture of f number although for a camara obscure
this does not matter much as long as the image can be seen on the "screen" upside down and reversed left to right.
andy
