Determining Pinhole Size and Exposure
The size of pinhole
you need depends on the kind of effect you want to get.
Many of us calculate the "optimum" size and
then depart (or not) from it, in order to experiment.
There are many formulas to calculate the
"optimum" size. Optimum, in this case, means
the hole that gives us the "sharpest" pictures.
Incidentally, the sharpest pictures may or may not be the
"best" pictures for you. The formula I use is:
where Pinhole Diameter and focal length are expresses in inches
where Pinhole Diameter and focal length are expresses in millimeters
Once you know the
size of the pinhole aperture you will need, find the
f/stop of your camera by dividing the focal length or
distance pinhole to film plane by the diameter.
Obviously, both values must be expressed in the same
More likely than not, the f/stop won't coincide with a full stop. Since the progression of f/stops is not linear, you would need a math formula to find where exactly in between full stops the f/stop of your pinhole camera is, but due to the "imprecise" nature of the pinhole practice, that is not really necessary (for some the more imprecise the better, some others want it imprecise but controlled to certain extent, others want to know how to make it precise so they depart from that precision and others like it very precise, period! No pinhole practicioners in the latter category though!).
I would suggest you approximate the calculated f/stop to the next full stop (unless is really close to lower one). The reason is that, pinhole exposures are more likely to be under than over exposed. Once you know the full f/stop of your camera, it's time to make some pictures. You then have to find the exposure your scene needs. Do it by whatever method you want. I use 2 methods. The first is by applying the "Sunny16" rule, which says that under sunny/bright conditions the exposure needed is f/16 and 1 / (ISO film speed). For instance, if the film is ISO100 the exposure needed would be f/16 and 1/100 secs. The second method is taking an actual light reading of the scene. Sometimes I use a handheld meter, other times I use my 35mm camera metering system.
I want to state at this point that exposure in photography is given by a pair of values, they are the aperture or f/stop and the exposure time, let's call then "f" and "t" respectively (we will use this letters later).
Once you have the exposure that your scene needs, by means of "Sunny16" rule or taking an actual reading with a light meter or camera, you have to find the equivalent exposure values for when the f/stop is the one of your pinhole camera. Let's call this new pair of exposure values "F" and "T", respectively (notice they are upper case letters and that "F" is the actual f/stop of your pinhole camera). You then start to double "f" until you get a value that is equal or bigger than "F". If equal, the number of doublings multiplied by 2 is the number of f/stops separating "f" from "F". If bigger, the number of f/stops between "f" and "F" is the number of doublings times 2 minus 1. The new exposure time ("T") will be obtained by doubling the time "t" as many times as stops separate "f" from "F". It is more difficult and cumbersome to say or write it, than to actually do it.
Let's use an
example, let's assume we have a 6" focal length
camera, we are shooting under bright sunny open skies and
that we are using B&W paper as our "film".
Let's mention here that B&W paper has an average
equivalent to film speed of ISO-6:
When shooting outdoors, you have to watch for changing light conditions during long exposures as you may have to adjust the exposure time a little bit to compensate for those changing conditions. When I was making DOOR (using B&W paper as negative material), the uncorrected exposure time was 8 minutes. The multiplier according to the table above is 5 for total corrected time of 40 minutes!!! A big cloud passed by during part of the 40 minutes, that made me extend the exposure time to 55 minutes to compensate. During this time, I also exposed my body for 55 minutes to 19 degrees bellow the freezing mark!!
Hope this can help you in your pinhole practice.
For comments, corrections, questions and suggestions, please contact the author.