1/300 sec @ F/16 on a Nikon with an FP shutter will give less exposure than 1/300 sec @ F/16 on a Contaflex with a leaf shutter.
Interesting.
A focal plane shutter will expose the film at short exposure times by moving an open slit across the film plane (vertically or horizontally). Since the shutter is close to the focal plane (what's in a name), there's virtually no intermediate zone between open and closed shutter. Each spot on the film/sensor virtually instantaneously 'sees' light as the slit arrives and then goes dark again once the slit leaves that spot. The light flux intensity vs. time plot would look something like this:
For a slow speed, the above plot will also be true for a leaf shutter. Take e.g. a 1 second exposure: the opening of the shutter leaves is virtually instantaneous in relation to the long overall duration of the exposure, and the process of opening and closing of the leaves won't affect the exposure much. It's different for high speeds, where a leaf shutter will start to show this kind of behavior:
Keep in mind that what matters is how much actual exposure each bit of the film/sensor receives: the green area underneath the plot.
Now, in your post, you seem to make an assumption of the shutter speed being the time the shutter
starts to open and
finishes closing. I don't think that's accurate, because shutter manufacturers have been aware of the above issue for over a century, and they figured out long ago that in order to maintain reciprocity between exposures (i.e. a 1/500 exposure is really half as much as a 1/250, which in turn is half as much as 1/125, etc.), they would have to take into account the opening and closing of the aperture blades. Consider this:
The actual shutter speed for a fast time on a leaf shutter will
not be T2
a-T1
a, but rather T2
b-T1
b. Of course, there's the practical challenge of actually making getting this exactly right in a small mechanical device that should function reliably for decades under adverse conditions, but decades and decades of innovation in mechanical engineering have resulted in rather reliable mechanical leaf shutters that do reasonably well up to reasonably fast speeds.
Now, the problem with the aperture is that if the aperture is extremely close to the leaf shutter (it's evidently never close to a focal plane shutter), a very small aperture opening (large f/number) would block a large part of the light falling through the open shutter. This would result in small apertures (large f/numbers) giving relatively
more exposure (not less, as you suggested) at such apertures compared to shooting wide open. However, in a practical design, the aperture is always a little bit offset from the shutter (which optically is a compromise of course) - and even a little bit of distance between a small aperture and an open shutter will still allow a whole lot of light to pass through and reach the film/sensor.
So, to the extent your argument has theoretical merit, it's in the opposite way of what you suggested.
In reality, I think the most important development around the time of the ISO revision standard is exactly what you mentioned: the introduction of new camera and shutter systems with far more accurate and consistent shutter speeds (or rather: effective exposure), across the shutter speed range and over the equipment's service life. This meant that a little less leeway was needed in allowing for natural variations in equipment (which effectively reduced, and quite dramatically at that), allowing for the same film to be rated a little higher because it was easier/safer to balance a little closer to the exact exposure instead of having to build in a massive fudge-factor to allow for Ansel's well-maintained leaf shutters used at half a second all the time vs. aunt Edna's Medalist shutter whizzing away at a sluggish (?) or rapid (??) 1/50 during family picknicks.
This, and there were changes to basic assumptions underlying sensitometry such as
a change from 2700K to 4700K as a sensitometric starting point, but these were also aimed at improving consistency between different types of equipment and workflows rather than accounting for some inherent problem with leaf shutters. Btw, if they were so problematic, I'd have expected Hasselblad etc. to have left them behind long ago. Apparently they didn't see a necessity for it.