Maybe by the time I wake up tomorrow, this endless discussion will be over.
On Apr 1, 2014, at 11:14 PM, James Schenken wrote: So, putting vignetting aside, let's deconstruct how the image is actually formed by a lens-leaf shutter combination.
For discussion purposes only to simplify the thinking,
Assume the shutter moves in fixed increments instead of continuously like analog.
Then further that the increments are exactly 20% of the area of the maximum aperture.
And, here is a real stretch, assume that the time for each increase in opening is the same.
So, the exposure goes something like this for max aperture
1st increment - 20%
2nd increment - 40%
Etc. until fully open and then it reverses.
For a small aperture
1st increment - 100%
2nd increment - 100%
Etc. just as before.
So on average, the max aperture exposure is 20% of the sum of 20, 40, 60, 80, 100
But for the small aperture the exposure is 20% of the sum of 100, 100, 100, 100, 100
This is a gross over simplification of what really happens but serves to illustrate the problem of exposure variation in leaf shutters as a function of shutter speed and aperture. I suspect that the opening speed of a leaf shutter is the same across a wide range of nominal speeds. There is some minimum shutter speed that corresponds to the open and close sequence with almost no delay at the fully open position. To the extent that a shutter is operating slower than that theoretical minimum, the difference attributed to aperture size should be reduced.
But with real shutters and lenses, the difference is much less than the example suggests. Real shutters start off slow, get up to speed, and then slow down as they open, delay a short time, and then reverse and do the same thing until they close.
Better ones spend less time opening and closing and therefore more time fully open than cheaper ones. I seem to remember at least one shutter that used rotating blades so there was no slowing down around fully open. That should have reduced the problem some.
Art Faul
The Artist Formerly Known as Prints ------ Camera Works - The Washington Post
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