I'm just curious as to what factors drove the design of early (pre-Copal) shutter speeds that are not in 2x steps, e.g., 1, 2, 5, 10, 25, 50, 100, 250, 400. I bet somebody here knows. Could it be a result of manufacturing precision capability (which I doubt)?
I don't know the reason, but you didn't go back far enough. Several of my old shutters have only 25, 50 and 100. Obviously these have a factor of 2. When escapements were introduced, I have forgotten by whom, the speeds below 1/25 began to appear. Why 1, 1/2, 1/5, 1/10 I don't know. Obviously the first two have a factor of two and the fastest two do also. Only the ratio between 1/2 and 1/5 does not comply.
I must say that when I began photography in the 30's all shutter speeds were based on the European standard. I always computed them a having a factor of 2x, and the exposures were close enough. When one takes into account the inaccuracy of most shutters, the small difference is of no consequence. Without checking I estimate that abut 2/3 of my 20+ shutters have the "old" speeds ad cause me no problems.
Interestingly enough, it is probably more useful for speeds below one second to fall into a geometric series than it does for speeds above 1/50 or so. The shutter does not go from completely closed to wide open instantaneously, and while the blades partially obscure the opening, they effectively reduce the lens aperture. This affects exposure, but for low speeds the effect is negligible. So ideally the shutter would deliver exposure in some geometric series, and only "effective" shutter speeds at the high end.
This was one of the reasons that the Speed Graphic and other press cameras had focal plane shutters; stopping motion requires a fast shutter, but at 1/1000 a leaf shutter would also act like a continuously variable aperture stop---until it self-destructed from the forces needed to start and then arrest the motions of the parts.
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