Originally Posted by
Nathan Potter
Well, roughly speaking there is an optimum aperture for pinhole resolution determined somewhere in pre history (actually by Petzval I think) that goes as D = (2 X f X wavelength)^1/2; that is D = the square root of (2 X f X wavelength). where:
D = diameter of the pinhole
f = the focal length (pinhole to film distance)
wavelength = about 0.5 µm
Following this relationship will provide you about the optimum resolution.
The effective f/no is simply the pinhole to film distance/pinhole diameter.
You have a further constraint in that you wish to minimize vignetting. Since there is no lens I think you want to say minimize light fall off at the edge of the film. This means you need to use a longer focal length depending on the size of the film you wish to use.
I'll, for example, assume you will use 4 X 5 inch film and further assume that to minimize light fall off you want to use a FL of 300 mm. (roughly a 2X tele).
Now for optimum resolution we have in mm. units D = (2 X 300 X 0.0005)^1/2 which equals about a 0.55 mm. (550 µm) diameter pinhole.
The effective f/no. of the 0.55 mm. pinhole is f/pinhole diameter which equals 300/.55 = f/545
Use f/545 to calculate your exposure for six weeks.
Now that is a long exposure so I'm guessing you will need to use a set of neutral density filters if you intend to use an open pinhole for the full time.
Play with my numbers for a different set of conditions - and good luck.
Nate Potter, Austin TX., Chatham MA.
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