To calculate the exposure time for pinhole camera, we first of all need to know the f number of the pinhole camera. Unlike normal cameras, this value does not change because the hole remains the same size which therefore simplifies the final calculation.
Pinhole f Number
This is the distance from the film divided by the diameter of the pinhole. Let’s assume that our pinhole camera has a focal length of 25 mm and the diameter of the pinhole is 0.18 mm . Using the information above, 25/0.18 = 138, so our aperture f number is 138
Pinhole Exposure Calculation
The high f numbers found on pinhole cameras in the majority of cases are not available on light meters. In order to get round this problem, we 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.
We do this by dividing the f number of the pinhole camera by the f number set on the light meter. We then square this number and use it to multiply the measured exposure time.
For example, if the light meter which is set to f/22 gives us a reading of 1/30 second, the calculation for our pinhole camera with an f number of 138 would be (138/22)2 = 39.3. The indicated time by the light meter has to be increased 39.3 times, resulting in an exposure time of 1.3 seconds.
Pinhole Exposure Charts & Exposure Guides
I have compiled a series of pinhole exposure charts for some popular pinhole f/numbers
The charts include pinhole exposure times for pinhole cameras with a focal length an f number of:
- 25mm f/138
- 40mm f/158
- 40mm f/235
- 50mm f/176
- 60mm f/207
- 75mm f/216