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Lesson Plans


Re: Pin hole cameras

[ Thread ][ Subject ][ Author ][ Date ]
wrapf (wrapf)
Wed, 18 Mar 98 17:23:27 EDT


>From many years of practice, the time of exposure given a hole of about .02 of
an inch is about 1 minute using RC Glossy photographic paper as a negative.
If the pin hole is very small .015 of an inch about 2 minutes is required.
Larger holes require less time. The shorter the time, the greater the chance
of error. if 30 seconds is the correct exposure, and one mis-times only 20
seconds the percentage error is very large. If the correct exposure is 2
minutes, and the error in timing is 10 or 15 seconds, the difference in
negligible.

The best pin holes are created by taking thin brass shimming and pressing a
"dimple" in the center. Then with a fine emory board, or sharpening stone,
grind down the raised dimple to open up a small round opening.

Exposure is dependent on the size of the camera (distance from lens to film),
the size of the pin hole, the sensitivity of the emulsion, and the brightness
of the ambient light. A lot of variables! Students learn a lot by having to
figure it out. Trial and error is a wonderful teaching tool.

If you want to be more exact, you can figure the f/stop of the lens.
Calculate the diameter of the pin hole and divide that into the distance from
the hole to the emulsion. The dividend is the f/stop. Knowing the f/stop
(usually around f/200) and a good hand held lightmeter, you can actually take
a light reading and determine the shutter speed. A good question for students
to ponder is "how do you measure the diameter of a pin hole?" With experience
I've learned that the average pin hole, is .02 of an inch. A large pin
hole, the diameter of a pin shaft is .025" and a small pin hole, (just the tip
of a pin) is .015".

View finders present another problem for students to figure out. You should
have your students construct a view finder to match what the camera "sees."

Pin hole cameras and their use are fantastic. So many teaching possibilities
and opportunities. Good luck and have fun.

Bill R.
Souhegan High School Amherst, NH