My 4x5 pinhole parameters:
Pinhole to film distance ( "focal length"): 2.44 inches (62 mm)
Exposure Mask Area: 3.9 x 4.75 inches (100 x 120 mm) Diagonal: 6.15 inches (156 mm)
Diagonal Field of View: 103º
Pinhole diameter: 0.012 inches (0.30 mm); about f/210
Pinhole Constant: 1.65
f/22 Exp Multiplier: 88
Weight: 550g (~1.22 lb) with filmholder
If the "coverage" dimension you state is coming from Mr Pinhole, I think it's rather pessimistic but he doesn't actually describe the criteria. I recall it told me the numbers above were good for 4+ inches, vs the 6.15 I'm using. Yes, there is some fall-off away from the center but it's not that awful!
(Backing up from that link will get you to all my pinhole stuff to date, such as it is. )
Last edited by DWThomas; 07-09-2013 at 12:23 AM. Click to view previous post history.
If you want to go crazy with the math, here is a thread to read all the way through:
Some months ago I started to use these formulae to try to find the shape of a surface that would have constant illumination ( instead of a flat film surface ). The shape accounts for the off-axis squint, the "tunnel" caused by the thickness of the pinhole material, and the distance to the film. I'm not convinced it really takes into account the shape of the diffraction blurring all that well. I haven't gotten back to it but the shape is somewhat vase-like, at first bulging outward and then diving inward toward the pinhole itself. It makes sense: as you move away from the center of the image, at first the distance is the dominant effect, requiring you to move upward closer to the pinhole to compensate, but as you move upward, the squint becomes more pronounced causing the radius to decrease.
( My undergrad degree was mathematics, and I've been a member of the Mathematical Association of America for almost 30 years now... so this sort of thing is a great temptation for me... )
But. Recently I've had to catch myself making things too technical and complicated. So I pulled back from all of this. I don't want to mess up the simple joy of finding the picture, placing my pinhole coffee can or cardboard box, and then watching the magic of the image coming up in the developer. To me the idea that the light from the scene landed directly onto the paper to make a permanent captured image is just wonderful. So these days I'm trying to keep things more about "play" and less about the technical part. Not that the technical part can't be fun, but there is a balance to reach!
So my advice: build the camera and have lots of fun making wonderful pinhole pictures with it!
DWT - yes I was using Mr. Pinhole as the calculator. I'm not really opposed to falloff thogh. I mean, I'm using paper negatives in a pinhole. I'm not going to get a tack sharp image anyways. Thats what the RB67 is for!
Ned - I tend to over think things too, especially in the conceptual phase (hence the size of this thread, and no pinhole built yet).
I might just go with DWThomas's numbers and go from there. Just 2 more questions though.
1- If I'm shooting for a .30mm pinhole, what is the best method, and how should I (somewhat) accurately measure?
2- what is the f/22 exposure multiplier?
1) I use the dimple and sand method, generally in one or two mil brass shim stock. I use a sewing needle and work against a scrap of mat board to produce the dimple, then #400, or even #600, wet-or-dry sandpaper, wet, and try to "sneak up on the size." First rule: it is much easier to make the hole a little bigger than a little smaller!
Originally Posted by edcculus
Thanks to some quasi-professional activities four decades ago, I have a little 50x hand-held microscope with a direct measuring reticle to check small dimensions, so I use that along the way. I believe others use a scanner or enlarger to try to measure.
2) That f/22 multiplier is a handy output from Pinhole Designer. You take the light meter reading of shutter speed at f/22 and multiply it by that factor. P.D. saves doing the math to get the multiplier, which is necessary because few meters go to f/275 or whatever. I usually generate a pocket-sized table for the multiplied shutter speeds to avoid calculators in the field. P.D. can even output such a table with or without reciprocity compensation for a number of films. Unfortunately it hasn't been updated in a long time, so some newer films are not in it. But all that stuff can be generated with generic tools (or pencil and paper!!!)
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Awesome Dave, thanks for the quick reply!
I work in the Graphics Arts industry, so i MAY have a loupe at work that has measurements on the base. It probably won't be powerful enough or measure small enought though. I'll probably have to do the scan and measure method.
I make mine just like Dave.
Until recently, mine were all measured with just a hand magnifying glass and a ruler with mm markings on it. You can estimate the size to within about .1mm that way. Sometimes lately I've used my enlarger to project a circle.
Another trick is to hold the pinhole up really close to your eye and look through it toward something bright. You will be able to tell if the hole is round and if there are any largish burrs. ( You might also see "floaters"... cells floating around in your eye's vitreous humour. )
oh wow I wish I could find an 8x10 for that price over here on eBay UK.
Originally Posted by rbeech
You can at least triple and usually quadruple it for the odd one that comes up
Hmm yes, I followed ePrey auctions for a while last spring and the average was about $32 for wooden ones and $52 for the newer more modern ones. The ones that went for less typically had missing latches or some dubious looking wear on frame or dark slides. Those could still be usable but I like to at least start with "nice" stuff!
Thats kind of why I went for 4x5 right now :-)
I got 2 wooden holders from a member in the classifieds for $10 including shipping.