Calculating the pinhole exposure times can sometimes be frustrating due to the small apertures (high f/stop) values we are using.
Before we can determine the pinhole exposure times, there are a few known values which we need to establish.
- Size of Pinhole
- Focal length
- Aperture Value
- Exposure Factor
- Exposure Time obtained from the exposure meter.
What I am about to describe does not take into account any reciprocity failure. Depending on the type of film you are using, you will need to add extra time to take into account reciprocity failure.
This is the size of the actual Pinhole
The focal length of the camera is determined by measuring the distance from the pinhole to the film plane
Aperture Value (f stop)
The f stop is calculated by dividing the cameras focal length / diameter of the pinhole
The exposure factor is where the magic happens and is used when we need to obtain the correct exposure time. The formula we can use to obtain the exposure factor is, (camera f stop / light meter f stop) ^ 2
* Actual Exposure Time obtained from the exposure meter.
Light meter time multiplied by the exposure factor.
* For exposures greater than 1 second, you will probably have to add extra time for reciprocity based on the film you are using. The two main films I use are, Kodak Tmax and Fuji Acros. Fuji Acros is one if not the best choices of film for reciprocity because there is none up to 2 minutes and even after 2 minutes it only requires an extra 1/2 stop.
Based on the information above, let’s look at a typical example of calculating the pinhole exposure times for a Zero 2000 6x6 camera.
From the Zero 2000 website, we know that the size of the pinhole is 0.18mm, the f/stop is 138 and the focal length is 25mm so all we need to do is calculate the Exposure Factor.
The formula is (camera f stop / light meter f stop) ^ 2 and we have decided that we are going to take readings from our light meter set to f/22.
(138/22) ^ 2 = 39.3 This means our exposure factor is 39.3
All we need to do now is take the reading from the light meter at f/22 and multiply it by 39.3 to give us the correct exposure at f/138 (excluding any reciprocity)
Say our light meter reads 1 seconds at f/22. Multiplying 1 x 39.3 = 39.3 seconds.
Exposing for 39.3 or 40 seconds will give you the correct exposure based on a pinhole camera having an aperture value of f/138.