Shutter efficiency

Hi.

I bought the Calumet shutter tester. In testing leaf shutters they

wrote that when your aperture is wide open you have a speed and when

stopped down you get a slower speed, because of the shutter blades. I

resumed what they wrote hoping someone in the list has a similar

tester and could enlighten me on this question. I made some testing

and didn't notice a difference, at least visually, probably with a

densitometer I could see the difference.

Thanks,

Jacques

Shutter efficiency refers to the fact that it takes some time for the shutter blades to move from the fully closed to the fully open position. If you define the shutter speed as the interval from the time when the shutter blades just let some light through to the time just before the blades once again completely block the light, you will get one speed. If at the same shutter setting you define the shutter speed as the interval between the time when the blades are completely open to the time when the blades start to close again, you will get a different speed. The first definition will give a longer shutter opening time than the second.

For slow shutter speeds the percent difference between the two ways of defining the speed is small and this issue hardly matters. For the shutter speeds typically used in LF photography, such as 1/60 to 1/15 with a Copal shutter, this isn't an important issue. The issue matters for shutter speeds close to the fastest (shortest) that the shutter can produce.

This means that the time the shutter is open (its speed) depends on the aperture that you have set the lens. For small apertures (large f-numbers), the first definition above is approximately applicable. When the lens is nearly wide-open, the second definition is more applicable.

What you really want to measure is an effective opening time, a short of average time based upon full vs partial admission of light, counting times when the shutter is partially open with less weight than when the shutter is fully open for the aperture that you are interested in. Obviously the inexpensive Calumet shutter is much too simple to calculate an effective speed.

The simplest way of using the Calumet shutter, with a light placed close to the lens, will probably cause it to start measuring the shutter as open when the blades have just started opening. This will give a measurement close to the first definition above. The Calumet shutter tester comes with some directions for positioning the light a bit further away so that it doesn't begin measuring the shutter open time until the blades are substantially open. This will give a reading somewhere between the first and second definitions above, and a reading that is probably fairly accuracte for using a lens with medium apertures.

In summary, these issues about how to measure the effective opening time of a shutter considering the lag of the shutter blades probably don't matter very much for most LF photographers. The effect is largest for when lenses are used wide-open or nearly so, and when the shutter is used at its fastest (shortest) speed or nearly so. Both factors are typically but not always absent in LF photography.

Jacques,

Accuracy and efficency of leaf shutters is described in Ansel Adams' book "The Camera", on pages 88-91. It has to do with a small aperture being completely uncovered earlier and completely covered later than a larger aperture, say, wide open. Stopped down to 16 or maybe even 11 and smaller, the difference would be negligible. Also, at the faster shutter speeds, the more the aperture size will make a difference.

Ansel said that for shutter speed testing, and I quote him. "In measuring the effective "speed" of leaf shutter settings for view camera lenses, I select f/16 as the pivotal lens stop. Evaluations made at this stop are usually within about 10 percent, the equivalent of 1/7 f-stop, a very acceptable degree of accuracy."

Hope this helps.

DG