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dpotter2113
2-Jul-2011, 19:41
Hi guys,
I'm planning on constructing an 8x10 pinhole camera this summer and before I begin the endeavor I wanted to get some math down to make sure that everything is going to work out well.

Here's the stats I'm working with:

Focal Length = 150mm
Pinhole Diameter = ~0.357mm
Film Size = 8x10 paper, so 203mm x 254mm

What I'm trying to figure out right now is the size of the Image Circle to make sure that I'll get decent coverage on the paper. I'm looking to avoid any vignetting. I've been scouring google for a formula to calculate the image circle size but have come up pretty empty handed. Any help you guys can offer would be awesome. Thanks!

Jim Jones
2-Jul-2011, 20:32
Pinholes have a less well defined image circle than most lenses. Vignetting with ideally constructed pinholes follows the fourth power of the cosine law. You shouldn't have a serious problem with a 150mm focal length on an 8x10 cameras. You would lose about two stops of illumination in the corners. Some pinhole photographers might go shorter than 100mm, especially with the use of film rather than paper.

The topic of ideal pinhole diameter has been the subject of much discussion for over a hundred years. An active pinhole site, f295.org (http://www.f295.org/Pinholeforum/forum/Blah.pl?) , has covered it and pinhole camera construction often.

Jim Noel
3-Jul-2011, 10:01
I use a pinhole at a distance of 75mm from the film. Edge fall-off, but no other problem.
S for ideal size for the pinhole, I have 17 charts made by 17 different "experts". No two are alike. You pay your money, or labor, and take your chances.

Emmanuel BIGLER
3-Jul-2011, 12:18
Hello !
I prefer to speak about 'distance of projection' instead of 'focal length' since in a pinhole camera you do not want to bother with focal planes and focusing ;-)

Jim Jones and Jim Noel have said everything, you should go on with your project !

150 mm means that the distance is about half of the diagonal of the format (about 300 mm, 12", for a 8x10" format)
This correponds to a maximum angle of 90 (plus or minus 45), a simple diagram hand-drawn on the back of an envelope is the only maths you need.
To the best of my knowledge, 90 is an angle that can be achieved easily with a good pinhole. Simply remember that slanted rays above +-45 could be blocked if the pinhole thinkness is equal to the pinhole diameter. If the pinhole is built on a thin foil, thinner than the pinhole diameter, in principle you can reach view angles above 90.

Regarding the best pinhole diameter corresponding to a certain projection distance, you should feel free to take whichever you like with a tolerance of plus or minus 30%. I mean : take 0.3 or 0.4 mm for a theoretical value of .357, you'll have hard times to tell the difference in image sharpness. However taking into account that the image quality is worse as soon as you depart from the best pinhole diameter, I would favor a slightly wider aperture in order to get slightly more light and shorter exposure times.
When using a smaller aperture than the optimum, you loose both on image sharpness and available light reaching your film ! definitely a bad idea ;)

Regarding the optimum diameter, I've been convinced that it actually exists since I've seen 3 images shown side-by side in Leslie Stroebels's book "View camea technique" ; in 8x10" format, one is shot with the optimum pinhole diameter, the second with half this value and the third with twice the optimum diameter. Stroebel's images show the evidence that the best is the best ... but you are allowed to cheat by 30% ;-)

Have fun ! pinhole photography means absolute freedom, and in 8x10" format, images can be very nice.
And if you are not afraid of long, long, exposure times, you could experiment the Harman direct positive paper (ahem .. exposure index about 1 to 3 ISO ...)

Jim Graves
3-Jul-2011, 15:27
Take a look at this site: Link (http://www.pinhole.cz/en/pinholedesigner/)

dpotter2113
3-Jul-2011, 18:48
Thanks for all the info guys! And thanks for the link! That is an amazing little program.