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Apache1322
08-13-2005, 10:36 PM
I ran across some trays used for 11x14 paper. I was wondering what I could use for a pinhole camera that would hold 11x14 paper? Also, would the exposure time be changed dramatically because it is 11x14? I'm currently using 5x7 and an exposure of 1 1/2mins. is good.

ImageMaker
08-13-2005, 10:47 PM
Hmmm. Let's see, you could make a camera from black-on-black foam core board, or from plywood (even masonite), or other lumber products, or you could look for an old Samsonite or similar suitcase or briefcase, even an aluminum case (a Halliburton would be most appropriate, but they usually aren't cheap enough to drill holes in).

What happens to exposure depends largely on what you wind up with for a focal length. Exposure on the same paper, with an optimum pinhole, will be about the same at any given focal length regardless of the paper size (you'll likely wind up adding a little with larger paper, in order to offset the light fall off as the edges get farther from the hole, effectively overexposing the center to keep the edges from going black in the print). However, it's common to lengthen the focal length; as long as you keep the hole close to optimum for the focal length, exposure (for a given material) will increase proportionally to focal length -- that is, double the focal length and keep the pinhole optimum, you wind up needing exposures twice as long on the same film or paper.

Tom Miller
08-15-2005, 01:16 AM
However, it's common to lengthen the focal length; as long as you keep the hole close to optimum for the focal length, exposure (for a given material) will increase proportionally to focal length -- that is, double the focal length and keep the pinhole optimum, you wind up needing exposures twice as long on the same film or paper.Shouldn't doubling the focal length quadruple the exposure time, based on the inverse square law? I've found quadrupling exposure between my 3.5" and 7" cameras works perfectly.

And, Apache, try using an old fashioned potato chip tin. Many of them hold an 11x14 sheet of paper perfectly. It is the same as using the classic oatmeal box, except bigger negs and longer exposures. Matte paper works best to prevent light banding.

murrayatuptowngallery
08-15-2005, 01:25 PM
Alternative to Halliburton case is $20 aluminum toolcase at Lowe's, HomeDepot-type store, etc.

Comes with removeable tool holder board and perforated foam. I used one for a camera case, holds two folders, light meter, spotmeter, Blik rangefinder, an inclinometer & a couple boxes of filtes, shades, etc.

Murray

ImageMaker
08-16-2005, 01:48 AM
Shouldn't doubling the focal length quadruple the exposure time, based on the inverse square law? I've found quadrupling exposure between my 3.5" and 7" cameras works perfectly.

Unless I fouled up the calculation somewhere, no -- the thing is, the hole diameter gets bigger as the square root of the distance, so the hole area increases in direct propotion to focal length, if the hole is kept optimum. That means when you double the focal length and keep the optimum hole, you're letting in twice as much light but spreading it over 4 times the area, so you need twice the exposure time (because half the intensity).

If your 3.5" and 7" cameras give good exposures with 4x exposure on the longer one, it's either that both holes aren't of optimum size, or an effect of reciprocity failure -- add 1 stop to required time, you have to add another stop for reciprocity correction.

Tom Miller
08-16-2005, 11:33 AM
Hi ImageMaker,

You're right. I checked the calcs as well, and the area of the optimal pinhole doubles as the focal length doubles.

I'm using identical 400 micron pinholes on both cameras, making the exposure time exactly four times longer. This was an arbitrary choice; the optimal pinholes for 3.5 and 7 inches are 352 microns and 498 microns; using a 400 micron hole sort of split the difference.

Tom