To calculate the exposure time for the 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 working aperture f-number would be 138.

Pinhole Exposure Calculation

The high f numbers found on pinhole cameras in the majority of cases are not available on light meters. To get around 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.

Free 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 different focal lengths and f numbers.

(25mm f/138)  (40mm f/158)  (40mm f/235)  (50mm f/176)  (60mm f/207)  (75mm f/216)