[Fotoguy20d]

The other part to consider is that for lens shutters, the real exposure time is not easy to measure, as during the time the shutter takes to open and then to close the amount of light passing through varies and it adds to the light passing through when the shutter is open fully. The mechanical shutters were adjusted to take that into account, so it may seem like then run slightly slow if you just measure the time the center of the shutter stays open.

If you use a computer with software that captures and displays the change in voltage against a time scale (audio software for instance), it's a relatively easy matter to determine the area under the curve (integrate, if you remember Calc 101) and compare it to the expected result. Although doing this reasonably quickly will yield a somewhat imprecise result, it'll certainly be within a 1/2 stop or less without too much work.

Dan