Re: Calculating f-stop for pinhole camera

Hi Rune,

The way I have used a slide projector to measure the size of a pinhole is
similar to a method devised by Jay Bender that is in Eric Renner's book on
pinhole photography:

I bought a cheap C-thru ruler - about 6 inches in length. I cut this to
fit in my slide projector (the C-thru ruler is transparent). I project
this on the wall and move the projector back until it projects 10 cm into 1
cm of the other piece of the ruler (or any ruler). I am then able to mount
a pinhole into a slide mount and project it onto the wall, and measure the
diameter of the pinhole. Each cm is equal to .1 mm.

I haven't done it this way for a couple of years, since I got a (toy)
projecting microscope, so my numbers may be off - but the method works very
well.

For what its worth, the photos I took after accurately measuring my
pinholes were not necessarily any sharper than the ones I just guessed at
for years - I just had a better measure of their size. A clean hole seems
more critical than an "optimal size" one to me (IF "sharpness" is your
primary concern).

Gregg

At 04:04 PM 4/7/01 +0200, you wrote:
>When making a pinhole by means of a needle one usually know the pinholesize
>by the needle size used.
>I seem to recall that there is another way to do get to know the exact
>pinholesize but cannot remember the formula used. The method was described
>as follows.
>Mount the pinhole sheet in a slideframe, place it in a slideprojector.
>Then by measuring the distance from the pinhole in the slideframe and to the
>projector screen and the diameter of the projected pinhole on the screen, by
>means of a formula it was possible to find the exact pinhole size. Anybody
>out there that know which formula (s) to use??
>
>Rune Tallaksen
>Norway
>
>
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Received on Sat Apr 7 11:02:01 2001