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Pinhole exposures – quick how to, OK, not so quick

January 28, 2008

I’ve seen more than a few posts where people are trying to figure out how to take a pinhole photograph with their dSLR. A common problem seems to be that they are still expecting the dSLR to figure the exposure or at least give them a preview.

I don’t have a dSLR so I can’t diagnose that problem but I’m betting that the dSLR CAN’T figure the exposure and can’t give you a preview for the same reasons a film SLR can’t. There is no interconnect between the f-stop (your pinhole), and the camera’s meter any longer and it is just too dim to see through the finder. So quit trying to use it that way.

This will require some thinking on the operator’s part so get ready (FYI, you can pre-compute most of what you need and just carry a cheat-sheet). This stuff works for any pinhole camera. I’m making some simplifications here so if possible, always bracket exposures.

By the way, Google is your friend (and so is Wikipedia). If you don’t recognize a term, go look it up. Also, Google is very good at converting units so if you don’t want to know that 1 inch = 25.4mm and learn how to apply that, no problem. Tell Google “3 inches to mm” and it will tell you the answer.

Steps:

  1. Know your equipment. You need to know several things. The film speed (or equivalent film speed if you are using photo paper as your “film”). The pinhole diameter. And the focal length, that is the distance from the pinhole to the film (or sensor if you are using a dSLR). Come to think of it, you need to know one more thing, how to run your camera in 100% MANUAL MODE. 🙂
  2. For the pinhole and focal length from #1, you can now compute the equivalent f-stop. How? The f-stop is the ratio of the focal length to the diameter of the pinhole (or iris opening for a lens). Example: let’s say we measured the depth of the camera from the front of the mounting ring to the sensor (or the filter over the sensor, close enough) to be 35mm. And we measured the thickness of our pinhole-bodycap to be 10mm. This puts our pinhole at 45mm (close enough for our purposes) from the “film”. And we measured the diameter of the pinhole to be 0.012″. Notice I have different units, they must be the same units for the calculation. 0.012″ = 0.3048mm, call it 0.3mm f-stop = 45mm / 0.3mm = 150. f-stop=f-150!
  3. Figure the exposure correction factor versus f-16. Why f-16? Because next step we will use the Sunny-16 rule so just trust me on this one. What we want is a factor to multiply a later exposure time by to correct for our new pinhole fstop. This is done by: (fpinhole / f16)^2. “^” is raise to the power so we are squaring things. (150 / 16)^2 = 87.89. Let’s be simple at call it 88. We don’t need a jillion decimal places for these things.
  4. Sunny-16 exposure rule (Alert – Google for more info) says if we have a sunlit subject, strong shadows (sunlight over our shoulder) the base exposure is 1/ISO for an fstop of f16. ISO? That is the film speed (or equivalent for photo paper, but that is another post). So, what speed? If we were using an SLR I’d suggest 200 or 400 speed film for outdoors. So set your dSLR on 400 or even 800 if you can. Now back to the Sunny-16 rule. 1/400 at f16 (assuming you picked ISO400). Great, but our pinhole is f150, now what?
  5. Take the factor from step 3 and multiply that by the speed from#4. 88 x 1/400 = 88/400 = 0.22. You probably don’t have 22/100 setting on your camera but you will have something close. Maybe a quarter second? Again, each camera is different, but a “Real” camera would have the quarter second marked with just a 4 (1/4). Get as close as you can.

So, now take your picture. 1/4 second exposure is tough to hand-hold so use a tripod or brace the camera. That is unless you are looking for the blurred exposure effect. You can’t frame the shot in your finder but you can probably see the result on your LCD afterward.

The simple calculations above are starting points. You may find that you need a little more or a little less exposure. This is where a cheat sheet helps because once you get the baseline you can write down the base exposure (Sunny-16) and then write down what you need for other common conditions like weak shadow, hazy, full shade, indoors etc in terms of + or – stops of light. What, dSLR shooters aren’t familar with the concept of adding or subtracting a stop? Google and Wikipedia. Go learn it… 🙂

And now a word for film shooters. Schwarzschild. Film doesn’t continue to respond equally to longer and longer exposures, and this may also affect a dSLR sensor but perhaps not in exactly the same way. Referred to as reciprocity failure or the Schwarzschild effect. So for long exposures with a pinhole camera, usually those greater than 1 or 2 seconds using film, you have a second compensation factor to apply.

Most film manufacturers will give you a simple table that suggests how much extra exposure you need for times between 1 and 100 seconds. Some give you a chart and you can interpolate from that. And a few will give you the Schwarzschild coefficient and then you have to do some math. You can also Google for the coefficient or perform your own testing to find it. The testing involves some equipment you probably don’t have on hand so lets skip that for now. Roughly, a coefficient of 0.8 works pretty well for several different films so I’ll use it as the example here. There are astronomy websites where people have done quite a bit of testing on films for long exposures and they come up with other numbers but for all intents and purposes, they tend to center around 0.8. Lacking any other information, it is a decent starting point.

p = 0.8, the Schwarzschild coefficient.

t_new = (t_old + 1) ^ (1/p) -1, the magic equation

t_old = the number we came up with in step 5 above and t_new is what we will really use for our exposure.

Lets say, for example, that our fstop is f500 and we are going to use 100 speed film.

(500 / 16)^2 = 976.56, call it 977 is our first correction factor for exposure. Sunny-16 says 1/100 at f16, so 1/100 * 977 = 9.77 seconds. I’m not going to round this off to 10, keep it at 9.77 for the next step.

Apply the magic equation,

(9.77 + 1) ^ (1/0.8) – 1 (the order of operations is 9.77 + 1, then raise to the power then subtract 1).

(10.77) ^ 1.25 – 1 = 18.5. Now I’ll round this up to 19 seconds. So while step 5 above told me my exposure should have been nearly 10 seconds, it would have been underexposed because of the reciprocity failure of the film. I should try 19 seconds instead. If possible, I’d still bracket and do 19, and maybe 13 and 24 seconds.

One more time, a cheatsheet would help here. Set up a spreadsheet in Excel or OpenOffice and you are done.

One last thing, still haven’t told you how to figure out the angle of view have I? Can’t use your viewfinder so how will you know what is in or out of view of the pinhole? See next entry.

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One comment

  1. Hooray! This post is mightily useful, especially the bit about Schwarzschild. Last time I did long expos at night I used the sunny 16 rule derivative and my friend used his very good lightmeter (same camera), and the times we got were very similar. We both doubled our time to cover reciprocity failure, and it came out about right, but it was extremely hit and miss and there was definitely bracketing involved! I’ll try the above, and the cheat sheet suggestion.

    Thanks for your comments too, found them really useful, and will have a go on my next batch of developing! 🙂



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