Brett,
Many thanks, very interesting read. I had completely misunderstood the function of the 'lifting heel'!
Michael
Michael, until reading that, so had I. It had never crossed my mind, but after reading it, it makes perfect sense. The way the slit of the Contax shutter is pre-formed at the fast speeds is ingenious. But with any focal plane shutter, if you want the exposure to be even across the gate, you have to keep the velocity constant (possible with electronics I expect, but easier said than done with mechanical spring powered shutters). Otherwise you need to vary the gap to compensate. It's for this reason that when working on more conventional focal plane shutters with separately driven curtains, you're generally better off setting the tension so the first curtain gains on the second slightly, rather than the other way around. You're generally looking for an even slit across the gate, but the lesser of two evils will usually be a slightly increasing gap. If both the curtain velocity increases, and the gap decreases, the two may well combine to underexpose the far side of the gate.
The Contax curtains running off together at their pre-formed slit, assuming the clearance at the clutches is good, there's no question of the gap changing across the gate. It's prevented by the shutter design. Hence, the leather heel to increase the curtain gap slightly to counter the increased velocity near the bottom side of the gate.
This was all new to me until I read the linked article. I had, in the past, pondered the implications of varying velocity. But the Contax shutter is unique, and certainly one of the earlier vertical travel ones too. I'd reasoned (obviously, now, incorrectly) that either the velocity change across the shorter dimension of a 35mm gate was of less consequence than it would be over the more conventional horizontal, or that Zeiss had managed to stabilise the velocity across the gate enough that it was not a problem. Of course, even Zeiss are not immune from the laws of physics, and their way of managing the tapering of the exposure that would otherwise occur without the leather piece is elegantly simple (it may be the only aspect of these cameras that is!).
What it all boils down to is that the conventional wisdom which dictates that the Contax slit width does not vary at the high speeds because it's pre-formed isn't quite correct, because the leather is included to widen the gap so that exposure will remain constant. There you go. Fascinating stuff.
Cheers,
Brett
