[Thomas Bertilsson]

Brightness of scene, time, film rating, and aperture. Three of those four variables are needed to solve the exposure question.

So for example a sunny 16 front lit scene (EV 15), the normal setting for FP4 (ISO 125) at f/16 would be 1/125th of a second.

You are changing the aperture and need to apply the f/ number indicated. F/158 for the pinhole or f/55 for the zone plate.

To get close you can just count stops. From f/16 1-stop less light is f/32, 2-stops f/64, 3-stops f/125, 4-stops f/256.

The corresponding times count from 1/125 1-stop brighter is 1/64th, 2-stops 1/32, 3-stops 1/16, 4-stops 1/8.

So for the pinhole (f/158) in "sunny 16" (EV 15) with FP4 (ISO 125) somewhere just longer than 1/16th of a second would be the target.

The zone plate's target time would be a hair shorter than 1/32nd.

If you move from the sunny 16-EV 15 situation to something darker, say EV 10; you will need to adjust the speed 5-stops to let in more light.

So in an EV 10 lighting situation the time would adjust from the sunny setting at 1/32nd to about 1-second. (1/16, 1/8, 1/4, 1/2, 1)

Here's a good reference for EV numbers to estimate with.

Mark,

The scale goes, stop by stop:

f/16, f/22, f/32, f/45, f/64, f/90, f/128, f/180, and you have to, without counting reciprocity failure, double the exposure.

So, from f/16 you have seven stops less light to f/180, the closest full aperture to f/158. From 1/125th of a second that makes 1s.