Pinhole Photography Exposures and Reciprocity Failure

When I first got my pinhole camera calculating the long exposures confused the hell out of me.

I Purchased a Lerouge 66 Pinhole camera from Greenwich Cameras in London.

I remember asking the guy on the stall about exposure times and he gave me a handy little leaflet which was a rough “Guesstimate” of how to expose a 100 speed film. Here is what the guide said. 

For 100 Speed Film

• Bright Sunshine – 2s
• Hazy Sunshine – 4s
• Cloudy Bright – 8s
• Cloudy – 15s

And thats it.

These exposures depend on the camera you are using and in particular it’s pinhole size. My Lerouge 66 has a pinhole size of 0.2mm, and Aperture of f.150 and a focul length of 30mm. 
I wanted to know more and in particular various film speed calculations and of course, reciprocity failure times. 
I downloaded an app for my android phone called… wait for it… Pinhole!

It’s a relatively easy app to use and by far the only one I really understood. All you need to know is your camera’s set up. 
So within the app you input your Focul Length (30mm). Pinhole Diameter (0.2mm) F Stop (f.150) Film size (120) and the speed film. The app will then give you a range of shutter speeds and corrisponding f number. 
You can also save these settings. 
So for example. Here is my set up within the app for FP4. 

You can save this information and name it for easy access another time. 
Swipe left and you will be greeted with the exposure scale. 

If you press the F Stop button (top left) it will increase to the next stop. But I always leave it on f.5.6 and I meter for f.5.6. and whatever speed my light meter suggests at f.5.6 then I convert it to the pinhole speed on the right. It’s quite simple. 
Then on top of that speed you need to allow for any reciprocity failure. 
I have made myself a cheat sheet for my reciprocity failures on various films for this camera. You can usually find a films reciprocity failure compensation on the films data sheet online. Ilford have quite an easy chart for reciprocity failure. 

So on the chart above you need to work out ’to the power of’ on a scientific calculator. So for example if we look at FP4 we can see it says 1.26
All you need to do is use this simple calculation. 
If my metered exposure for FP4 at EI 125 and at f5.6 is 1/60th I look at the Pinhole Chart and it says convert to 11 Seconds. I now need to work out the Reciprocity failure. 
On a Scientific Calculator (I use an App) I input 11 (xy) 1.26 and it returns 20.518 933 468 365 3. In short 20.5 Seconds. And that would be my correct exposure for my scene. 

I have used this system for all of my pinhole photography and so far have had good results. I am not sure how accurate the app is but it seems to work quite well.