This is my quick cheat table if I take my pinhole with me.
|DETERMINING EXPOSURE TIME FOR PINHOLE CAMERAS |
You have to work with small apertures (high f numbers) and long exposure times and the reciprocity law failure (Schwarzschild effect) must also be taken into consideration.
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.
Many pinhole photographers simply use estimated exposure times. Also, many commonly used films have high exposure latitude and therefore are less sensitive to incorrect exposure.
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.
Using a lightmeter the problem is that the high f numbers on pinhole cameras are not available on light meters.
The 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 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)
For long exposure times, usually for exposures longer than several seconds, it is necessary to extend the measured time.
The majority of film stocks indicate in their specifications by how much the exposure times should be extended; if not you have to experiment.
Tips for correct exposures
Choose a material with high exposure latitude, this increases the probability of obtaining a useful negative. 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.
Indoors the times are very long, often more than one hour.
Usually, the only possible method to obtain a correct exposure is trial and error.
The sensitivity of the photographic paper is to be tested. The light meter should be set to somewhere between 2 and 10 ISO.
A good idea for simplifying exposures is to create a table for each pinhole camera and type of film stock.