View Full Version : Shutter speeds and aperature

Ed Richards
9-Dec-2006, 17:54
Just got a shutter tester off of Ebay as a first step to tightening up my exposure and processing. My lenses were pretty good up to about 1/60, OK at 125, and quite slow at 250/400/500. Interestingly, the most accurate across the board is a Linhof Symmar/Compur that is about 60+ years old. Since I shoot Tmax 100, I am never shooting about 1/60, so the faster speeds do not matter.

Got to thinking about aperture and wondering if the shutters are really slow. As the speed increases, the time spent opening and closing increases as a fraction of the shutter speeds. Since you test at small apertures, you get the longest speed. But I suspect that if you had the lens wide open or down a stop or so, and you measured the total light through, you might find that a shutter speed that measures slow is correct when you look at the total light, as you would for an exposure. So if you use a small aperture to compensate for the slower speed, you get an under exposed negative.

Thoughts on this?

Walter Calahan
9-Dec-2006, 18:13

Andy Eads
9-Dec-2006, 18:30
This is over a year since your post but it bears some attention. There is an effect caused by the combination of aperture and shutter speed on total exposure. One measurable characteristic of a shutter is how fast the blades open and close. This is called the efficiency of the shutter. If the shutter opens to full aperture slowly the total exposure is higher with the lens set at a small f/stop than at a larger one. Imagine this, a lens is set for maximum aperture but the shutter takes a long time to open and close. The shutter then acts as a moving aperture during the exposure interval. At long exposure times, the inefficiency of the slow shutter blades may be insignificant. But at short exposure times the slow blades mean that the lens is not open to full aperture for the full exposure. Conversely, using the same shutter, the effect is less at all shutter speeds when the aperture is set to a small opening. The inertia (or whatever is slowing the blades) of the blades will have little effect since the aperture will be fully revealed in little time. I fear this may be as clear as mud but I hope it helps. Andy

Andy Eads
9-Dec-2006, 18:31
Ed, Ooops. I read your Join Date as the post date. So, my response may be more timely than usual. Andy

Ernest Purdum
10-Dec-2006, 11:04
Ideally, a shutter would compensate for efficiency factors by exposing a little longer when large apertures were selected. This wouldn't be impossibly dificult for an electronically controlled shutter, but I can't imagine it being accomplished mechanically.

Fortunately, LF workers nostly work with rather small apertures so this is rarely a problem.

Lots of inaccuracies cause little problem, or may even be unnoticed, because a one stop difference is double or half the amount of light.