Shutter Speed Testing

Testing Shutter Speed

By Doug Criner

To obtain correct exposures, your film camera's shutter speed must be reasonably accurate. But measuring shutter speed presents some unexpected challenges.

Most small cameras have focal-plane shutters. There are two shutter curtains: the front curtain travels across the film and opens the lens, followed by the rear curtain which comes along behind and closes the lens. In effect, there is a traveling slit in front of the film, and the width of that slit depends upon the shutter speed setting. Different parts of the film frame are exposed at slightly different points in time, which causes the misshapen wheels in the photo of a moving bicycle.

For slow shutter speeds, it might be adequate to measure the time interval between the lens being, say, partially open and partially closed, ignoring the time required to open and close the shutter. For fast speeds, the lens might not ever be fully open, since the rear curtain starts to close before the front curtain is fully open. So measurement of shutter speed actually involves measuring the total amount of light energy falling on the film during the exposure. For example, if the shutter is set for 1/250th of a second, it needs to expose the film one less stop (i.e., one half the amount of light energy) than at 1/125th of a second. Therefore, one cannot simply set up a timing circuit started and stopped by photo-diodes.

A simple check is to expose a roll of film, photographing a uniform surface. For each exposure, increase the shutter speed by one stop and open the lens aperture by one stop. After the film is developed, all the frames should have roughly the same density. If not, suspect that the shutter needs to be adjusted. Usually what happens, particularly for a mechanical shutter, is that the faster shutter speeds fall out of calibration while the slower speeds stay reasonably accurate. For this reason, I'm particularly leery of using a camera's fastest shutter speed unless I've verified its accuracy.

The "cat's meow" is to use a special-purpose electronic shutter-speed sensor. Mine is a Camlogix SH-T2. By adjusting the sensor bias with the camera shutter open (on "B"), the speed measurement is accurate for fast shutter speeds.