Do pinhole cameras require "bellows extension factor"?
I'd like to build an 8x10 pinhole portrait camera with a "focal length" of 360mm. When making an exposure would there be any need to compensate for the long distance between pinhole and film? If so, how would I do that short of using precious film to test, or would that be my best bet?
The focal length divided by the pinhole diameter gives you the effective aperture. For example, 360mm/.5mm = f/720. So, the extension part is pretty simple. Where you will run into problems is with reciprocity failure due to the requirement of very small aperture needing very long exposure times. A reciprocity departure table for your film will give you some idea of the compensation needed.
Distance is, of course, a factor in determining exposure, however there is no such thing as a compensation factor for pinholes, since there is no focus at all, let alone infinity focus, 10M, 3M, etc. There would be no reason at all to apply it in the classical sense. The inverse square law always applies, however. What you do is figure a rough f stop by considering the length from pinhole to film and the diameter of the pinhole. If you change length by moving the standards closer or farther from each other, it will be more simple math to recalculate a new f stop than it will be to use the inverse square law as applied in the formula for bellows extension factor.
Last edited by 2F/2F; 12-24-2008 at 02:05 AM. Click to view previous post history.
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Here's a link to a pinhole calculator. I use it. It might help.
Other calculators are available on the left side menu. I rarely venture into that section.
"Lo único de lo que el mundo no se cansará nunca es de exageración." Salvador Dalí
The optimal size of the pinhole is found this way...Optimum pinhole diameter in inches = 0.0073 times the square of the focal length (in inches) . So for a 360mm (14") focal length, it is 0.0073 times 3.74 = 0.027" pinhole ...optimal for sharpness, I believe. So that is a type of compensation one does for extended bellows...the longer the focal length, the bigger the hole. Of course, one needs a way to measure/make a pinhole that is exactly, or close to 0.027"!!!
It is the very short focal lengths that can be problematical. I have several pinhole cameras made out of old 250 sheet boxes of 8x10 photo paper. The distance from the pinhole to the film directly across from the pinhole is 3". From the pinhole to the corner of the 8x10 film is 7". The old inverse square law says that I will have a little over two stops less light hitting the corner than the center. (works great if I point the camera at something that is dark in the center and brighter around the edges!). But at 14 inches, that will not be a problem!
PS...my avatar was taken with one of the photo paper box cameras (I have no idea what size the pinhole was).
At least with LF landscape, a bad day of photography can still be a good day of exercise.
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I use this to calculate the pinhole design: http://www.pinhole.cz/en/pinholedesigner/ it also gives you a table with reciprocity failure for different films.
Anyway, you don't have to focus with a pinhole camera, so you don't have that problem. The pinhole hole must be very precise in dimension and shape, usually a drill and other common perforating tools will give uneven edges, resulting in a some way distorted or uneven lighting of the film, the "best" pinholes are made with a precise laser cutting machine, but it depends on what you want to obtain.
The so called "optimal size" is a compromise that gives the "best" resolution with the least diffraction. If you made it smaller, you'd have sharper resolution, but more diffraction effects. To make it larger, you'd get more mush from the decreased resolution, but the diffraction would not affect the image as much. Of course, what "best" and "worst" mean are dependent upon what you want. I happen to prefer greater resolution with greater diffraction to a certain extent. This is because I enjoy shooting into the sun. Another aspect of the "optimal" is that it isn't the same for all wavelengths of light. Since different colors diffract more or less. Red diffracts less (that is, changes direction less) and blue more. This is why a diffraction grating works. Generally, a region in the green portion of the spectrum is chosen for these calculations.
Almost universally, these two factors are rarely considered each upon its own, but are assumed to be simply defects having equally degenerative consequences for the image, but the images you can get by adjusting the resolution/diffraction balance will not be the same. You get to choose your defect.
Technically, yes, if you chose to extend the bellows on a bellows-equipped pinhole camera, the focal ratio therefore changes, and you would have to compensate the exposure accordingly. But, given the near-unlimited depth of field of pinhole, there's no reason to extend the bellows for close-focusing, as in a glass-lensed camera.
If you are interested in setting up a pinhole camera so as to be optimized for close-up images, make the focal ratio smaller (i.e. the pinhole diameter smaller) than what most formulae recommend for the "optimal pinhole size", since these formulae, by and large, are assuming objects at infinity. At subject distances close to the camera, geometric effects of blurring the image due to divergence of the light cone through the pinhole (from non-parallel rays) overwhelms the effects of diffraction.
True, keeping the pinhole diameter the same and instead extending the bellows does end up making the focal ratio larger, but it doesn't improve image sharpness for close-in subjects, since that is a function of pinhole diameter. One way to make this practical is to use a removable pinhole lensboard with a selection of pinhole diameters to choose from.
On a related subject, I also use "improvised optics", like magnifying lenses, binocular objectives, fresnel lenses, etc, as camera lenses in a homemade nested box camera (or on my Speed Graphic). The way that I compensate for the so-called bellows extension is simply to measure the distance from the film plane to the lens, with a metric ruler I carry with the camera, and divide this number by the aperture diameter. I'm typically using homemade waterhouse stops, so the aperture diameter is a known value. This way affords me the true focal ratio at any bellows extension, which I then merely transpose onto my light meter to determine the recommended exposure.
For example, my 150mm binocular lens has a 50mm diameter wide open, operating therefore at F/3 when focused at infinity. But for close-in subject matter, the box camera's focal length may measure, say, 225mm, therefore the lens is now operating at 225/50 = F/4.5 instead.