I've been playing with a pinhole lens I made from a 0.35mm pinhole I purchased from EBay, a M42 metal screw on body cover, and a Nikon M42 to F converter. What I'm going to present is the math to determine the f stop of an pinhole lens and an easy way to use it with a light meter.

First of all the focal length is the distance between the point of convergence in your lens to the sensor or film in your camera. In the case of a pinhole lens that point of convergence is the pinhole. So in the case of the Nikon F mount the focal length of a pinhole mounted as I have is approximately 50mm, give or take.

So what does that make the f stop # for this pinhole lens? The f stop is equal to the focal length divided by the aperture. In my case with a 0.35mm pinhole and with a 50mm focal length its 143.

So how do I use that with a light meter since none of my light meters go up to f stop 143? If you do a 1/2 stop series spreadsheet starting at 1 you find 143 falls in between f stop 128 and the next 1/2 stop of 152.2. Rounding up to 152.2 I have found that there is a conversion factor between f2 and f152of 5400 so if your light meter gives you a reading of f2 at 1/60" multiply that shutter speed by 5400 resulting in 5400/60=90" By the same token a light meter reading at f22 has a conversion factor of 45. So if your light meter gives a shutter speed of 2 seconds at f22: 2"*45=90". Both readings are EV 8 by the way.

I'm certain that there are other ways to arrive at shutter values for such high f stops this is just one way. This method works for me and results in good exposures.

Comments welcome.

I've been playing with a pinhole lens I made from a 0.35mm pinhole I purchased from EBay, a M42 metal screw on body cover, and a Nikon M42 to F converter. What I'm going to present is the math to determine the f stop of an pinhole lens and an easy way to use it with a light meter.

First of all the focal length is the distance between the point of convergence in your lens to the sensor or film in your camera. In the case of a pinhole lens that point of convergence is the pinhole. So in the case of the Nikon F mount the focal length of a pinhole mounted as I have is approximately 50mm, give or take.

So what does that make the f stop # for this pinhole lens? The f stop is equal to the focal length divided by the aperture. In my case with a 0.35mm pinhole and with a 50mm focal length its 143.

So how do I use that with a light meter since none of my light meters go up to f stop 143? If you do a 1/2 stop series spreadsheet starting at 1 you find 143 falls in between f stop 128 and the next 1/2 stop of 152.2. Rounding up to 152.2 I have found that there is a conversion factor between f2 and f152of 5400 so if your light meter gives you a reading of f2 at 1/60" multiply that shutter speed by 5400 resulting in 5400/60=90" By the same token a light meter reading at f22 has a conversion factor of 45. So if your light meter gives a shutter speed of 2 seconds at f22: 2"*45=90". Both readings are EV 8 by the way.

I'm certain that there are other ways to arrive at shutter values for such high f stops this is just one way. This method works for me and results in good exposures.

Comments welcome.

First off, there is no point of convergence in the lens. Focal length is the distance between the lens and the focal point on the film or sensor, when discussing photography. Light rays do not converge within the lens, but some distance "behind" it.

The f-stop of a pinhole lens will be the distance between the pinhole and the photosensitive surface. Due to the optical characteristics of a pinhole, that distance can vary. So, if you know the diameter of the pinhole and the distance between the pinhole and the photosensitive surface the f-number is equal to that distance divided by the diameter of the pinhole.
--Bob