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DETERMINING EXPOSURE TIMES FOR PINHOLE CAMERAS

Determining the correct exposure time for a pinhole camera is truly a hard nut to crack. The situation is complicated by small apertures (high f numbers) and long exposure times, and in their calculation, the reciprocity law failure (Schwarzschild effect) must also be taken into consideration. Before I describe how to calculate correct exposure times, I would like to point out one important fact. Taking photographs with a pinhole camera is always something of an experiment and requires a bit of playing around. Achieving perfect results is not always the most important aim and certain insufficiencies in the exposure do not therefore lead to a fatal mistake. Many "pinhole" photographers successfully simply use estimated exposure times and leave the light meter at home in the drawer. Also, many commonly used films have high exposure latitude and therefore are, to a certain extent, less sensitive to incorrect exposure times.

However, if we want to minimise the risk of poor-quality photographs, it would be helpful to be able to calculate exposure times as simply as possible so that one has more time to concentrate on the photograph itself and also so the whole process does not become a mathematical nightmare. One option is to prepare a simple table for each pinhole camera whereby the time measured by a light meter can be quickly converted to the required time for the given pinhole camera and film stock. You can use the PinholeDesigner program to help you with the following calculation.

f number

In order to calculate an exposure time, it is important to know the f number of the pinhole camera. Compared with normal cameras, it does not change (the hole is the same size) and the calculation is simple: the distance from the light-sensitive material divided by the diameter of the hole. For example, the formula for a pinhole camera with a focal length of 100 mm and a pinhole 0.4 mm in diameter is: 100/0.4 = 250, hence the f number is 250.

However, the problem is that the high f numbers common on pinhole cameras are not available on the majority of light meters. The only way round this is to set the light meter to a different aperture, usually f 22, and then convert the measured exposure time for the aperture of the specific pinhole camera. This is done by dividing the f number of the pinhole camera by the f number set on the light meter; this number is squared and the result is used to multiply the measured exposure time. For example, if the measured exposure time for f 22 is 1/60 second, the calculation for our pinhole camera with an f number of 250 is: (250/22)² = 129. The measured time is increased 129 times, therefore the exposure time for the pinhole camera will be 2 seconds (rounded).

Reciprocity law failure (Schwarzschild effect)

Originally it was accepted that the photochemical change is caused only by the amount of absorbed radiant energy which is proportional to the sum of the amount of light and the length of time the material was exposed to this light. The relation between the photochemical reaction and the amount of absorbed energy is therefore directly proportional. However, research by several scientists, including K. Schwarzschild, showed that this reciprocal rule does not apply when light intensity is low. In reality, low light levels over a longer period have less effect than strong light levels over a shorter period, even though the sum of light intensity and exposure time is the same.

What does this mean in practice? For long exposure times, usually for exposures longer than several seconds, it is necessary to extend the measured time. The additional time is different for each type of light-sensitive material and for each measured time. The majority of film stock manufacturers indicate in their technical specifications by how much the exposure times should be extended; if not, then the only way to achieve correct exposures is experience.

Tips for correct exposures

Choose a material with high exposure latitude, this increases the probability of obtaining a useful negative despite certain mistakes during exposure. In general, conventional light-sensitive layers (which do not use T-grain emulsions) have a higher exposure latitude, such as Ilford FP4 Plus, and also the majority of commonly used colour negative films.

It is very difficult to set the correct exposure time for interiors where the lighting conditions are generally not so good. In most cases, the times are very protracted, often more than one hour. Usually, the only possible method to obtain a correct exposure is trial and error.

When it comes to setting exposure times, the use of photographic paper instead of negative material would require a separate chapter. The light sensitivity specified by manufacturers is measured in a completely different way than for film, and is unfit for our purposes. The sensitivity of the photographic paper should be tested. The light meter should be set to somewhere between 2 and 10 ISO.

Obviously, during exposure the pinhole camera must not be moved, otherwise the picture will be blurred. If the pinhole camera is light and cannot be fixed to a tripod, it should be weighed down.

As I mentioned previously, a good idea for simplifying exposures is to create a table for each pinhole camera and each type of film stock. The table for our example pinhole camera might look like this:

Example of an exposure table for a pinhole camera with f number 250

To take a photograph, just measure the scene to be photographed with the light meter set to f 22 and then, in the row for the measured time, look up the time for the given pinhole camera and film stock.