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

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