Okay, so you’ve built a pinhole camera and put some photographic paper or film inside. You’ve pointed the camera at your subject. Your finger is poised over the start button of your stopwatch. Now you need to know how long to leave the shutter open. This page contains a list of all the methods I know for calculating the exposure time. Choose the one you like best.
But First…
Do you know the f-stop of your camera? On a lens camera, you can change the f-stop by adjusting the aperture (how wide of an opening is made when the shutter opens). Of course the size of the opening on your pinhole camera isn’t adjustable – it’s just a hole poked in a piece of metal! So, the f-stop of your camera is a fixed number. To determine the number, divide the focal length of your camera (the distance between the pinhole and the paper/film) by the size of the pinhole. For example: the focal length of my camera is 2 inches and the size of the pinhole is .0103 inches, so the f-stop is 2 divided by .0103 = about 194.
If you don’t know the size of your pinhole, I’d suggest using Guillermo Peñate’s method of measuring pinhole size with a scanner. If you don’t have a scanner, you can hold a ruler next to the pinhole under a magnifying glass and go from there. I use a machinist’s ruler with markings down to 100th’s of an inch and that works really well.
Another way of finding your camera’s f-stop number is to look at the big chart on page 124 of Eric Renner’s Pinhole Photography book (which I highly recommend). In any case, once you calculate your camera’s f-stop number, write it on your camera so you don’t forget it. Okay, on to the methods of finding exposure times…
Method 1: Guessing
Yeah, guess. Open the shutter for some amount of time, develop the negative and if it’s underexposed (ie: the negative’s too white) then leave the shutter open longer next time. If the negative is too dark, then the exposure should be shorter next time. It’s that simple. Of course, you sort of need a starting point and you may waste some time and materials in the beginning. On the other hand, you don’t have to deal with the calculations and/or meter readings of the next two methods. Plus, once you’ve used the same camera for a while you’re able to make some pretty good guesses based on past experience.
Method 2: The Sunny 16 Rule
I’ll let the aforementioned Guillermo Peñate (one of the pinhole wizards on the Pinhole Discussion List) describe this method. Look in the middle of his Determining Pinhole Size and Exposure page for a description of the Sunny 16 rule. I’ve never used this method. It seems to require that you be able to visualize the progression of f-stops and make some calculations in your head – or at least carry around a list of f-stops that you can refer to. It may be better than the next method because you don’t have to own (or carry around) a light meter.
Method 3: Exposure Calculators
This is the method I use if I’m not guessing. I bring a light meter with me and use that to determine the exposure. If I’m using RC paper in the camera, I set the light meter for a film speed (ASA) of between 2 and 6. The only problem with using a light meter is that most of them won’t allow you to set it to a very high f-stop (which your pinhole camera probably has) (which your pinhole camera probably has), so what you need to do is take a reading for some arbitrary f-stop like f/22, then use an exposure calculator to determine the exposure time for your camera’s f-stop. I’ve seen some exposure calculators that you can use on the Internet or run on your computer but uhh, I’m not usually carrying around my computer when I’m taking pictures! This exposure-table-creator-thing at mrpinhole.com is pretty useful, though. It allows you to create a conversion table for your specific pinhole camera that you can print out and carry around with you.
Here are some exposure calculators that you can print out and piece together. Thanks to Nick Dvoracek for converting them to PDFs for us. Choose the one that you like best – they all do basically the same thing.
- Dr.-Ing A. Irmer’s Exposure Value Extension disc. This is the one I use. You only need to print out one copy of it, paste it onto a piece of cardboard, then cut it out and put the two pieces together with a clip of some kind.
- Richard Koolish’s disc should be printed out three times and cut up accordingly. This disc only works if you have a Gossen Luna Pro light meter since it contains the light readings from that specific meter instead of using the exposure times from the meter (like the disc above). Richard is also on the Pinhole Discussion List. He and I have taught a Worldwide Pinhole Photography workshop together at the Cambridge Center for Adult Education for years.
- Another calculation disc. This one was made by Brian Reynolds. One cool thing that he mentioned to me is that you can open the PostScript version (available on his website) in a text editor and tweak the values. Pretty neat.
…One More Thing
So let’s assume you’ve determined the number of seconds (or minutes – or hours!?) that your shutter should remain open. The only other thing to consider is something called reciprocity failure. Basically, film and photographic paper don’t work the way they’re “supposed to” when exposure times get very long – they require more exposure than you’d expect. If you’re using RC paper in your camera and the exposure time you came up with is more than a second (which it probably is), then you should take another look at the Determining Pinhole Size and Exposure page mentioned above – there is a chart that will help you adjust your times for this problem. If you’re using film, you should look at the data sheet for the specific film you’re using – it should have a similar chart to help you figure out how much to increase the exposure time.
Some other things to consider:
- Indoor exposures can be kind of hard to calculate so they might take some trial and error
- It’s possible to put a filter in front of (or behind) your pinhole for different effects. This will probably increase the exposure time and will likely require some more trial and error to come up with exposure times
- If you’re outside, you might have to adjust the exposure time while you’re taking the picture! If a cloud makes its way past your subject during what you though was going to be a 5-minute exposure, you’re going to need to keep the shutter open longer than 5 minutes.
- When you open the shutter, try not to move the camera too much – especially if it’s a short exposure time. Your image may come out blurry or it may give it “double-vision” (both of which can be neat, of course). If you’re using a piece of tape for your shutter and you don’t like the way it makes your camera shake when ripping it off, it might be worth trying to find a way to attach a cable release or something. Or maybe you just need to use some less-sticky tape?
- If your camera is very light, you can put something on top of it to keep the wind from moving it during the exposure. A brick or a shoe works.
- If you attach a 1/4″-20 (aka quarter-inch/twenty-pitch aka “quarter-twenty”) nut to the bottom of your camera, you’ll be able to attach it to a tripod to make it even more stable.
Good luck and have fun!
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