Pinhole camera exposure time calculator.
I made this to compute exposure times for a pinhole camera I made, using my regular camera as a light meter.
Usage notes: Set camera to spot metering, no exposure compensation.
In manual mode, set f-stop and ISO. I used f/2.8 ISO100 for a start. Adjust shutter speed until something dark is at 0 on the exposure scale. We want to expose for the shadows because the pinhole camera is "film" (actually photographic print paper).
In Aperture Priority mode, set f-stop and ISO, point at something dark and half press the shutter or do whatever you have set to trigger metering. The camera tells you the target exposure time.
Now we have: Meter f-stop Meter ISO Meter exposure time
The pinhole camera was built with an aperture made with a #10 sharp hand sewing needle. http://www.jjneedles.com/products/Regular-Sharp-Hand-Sewing-Needles-Sizes-5%252d10.html says that that makes the diameter about 0.53mm
We take that over to http://www.mrpinhole.com/calcpinh.php enter 6.36" focal length (20" half circumference / pi) enter 0.53mm pinhole diameter and get f/305 The pinhole could be as large as the next needle size up which is 0.61mm producing f/265
Our paper is Ilford Multigrade IV RC (5x7 for testing, final is planned to be 12.5 x 20) which the general internet estimates as ISO 6.
That gives us: Pinhole f-stop Pinhole ISO
Sample usage: pinhole.py 2.8 294. 100.0 6.25 0.01
f scaling factor: 11025.0
ISO scaling factor: 16.0
unadjusted exposure time: 0:29:24.000
adjusted exposure time: 4:50:39.917
In this example the camera says our exposure time should be 1/100th of a second. That's a nominal EV of 11. The unadjusted exposure time is about a half hour. The adjusted exposure time is almost five hours. The adjusted time here is likely quite wrong but the only equation I could find for the reciprocity breakdown adjustment of paper negatives was the one in the script.