Interesting Tidbit about exposure compensatiion with leaf shutters at small apertures

[Drew Wiley]

I've often thought about this. Then you've got the possibility of aged shutters which have drifted or become more erratic. But I simply grab a Calumet electronic shutter speed tester and test at all the speeds, at those actual apertures I'm most likely to use for that particular lens, and with multiple readings each instance to give me an idea of consistency too. I record all that for each lens; and if there are any significant errors, put a little note on the lensboard itself. But other than the highest speed setting in each case, it's remarkable how close all my shutters are to what they're supposed to be at typical usage apertures of the specific lenses involved, and how consistent those respective speeds are. Guess I have it easy, since I don't have anything older than a Copal 3S, and the rest are modern Copals.

So far, no rude surprises for me, even with fussy color chrome exposures. One more arcane "what if" which, for all practical purposes, I can ignore.

[Vaughan]

One would have to go a long way back in history to find the source of this alleged engineering blunder.

It's not an engineering blunder, it's a consequence that the shutter blades take a finite amount of time to open and close. Focal plane shutters suffer an equivalent consequence of finite curtain opening time which is image distortion when the subject is moving.

Note that if flash is used there s no error (since the exposure occurs only. when the shutter is fully open) and at typical working apertures like f16 or f22 the error is not significant. Even if the error was significant enough to affect transparency film, it would be consistent and would be picked up during testing so condensation could be applied.

[BrianShaw]

Vaughn… my tongue was in cheek while responding to the “why couldn’t they have done something to avoid this “problem”. Sorry that was not clear… I throughly using the term “alleged “ would telegraph irony. Will do better next time.

Great explanation you gave!

[Kiwi7475]

It's not an engineering blunder, it's a consequence that the shutter blades take a finite amount of time to open and close. Focal plane shutters suffer an equivalent consequence of finite curtain opening time which is image distortion when the subject is moving.

Note that if flash is used there s no error (since the exposure occurs only. when the shutter is fully open) and at typical working apertures like f16 or f22 the error is not significant. Even if the error was significant enough to affect transparency film, it would be consistent and would be picked up during testing so condensation could be applied.

“Blunder” is too strong a word, but the error can be partially designed out, and if it isn’t, then that can be regarded as a design flaw. “it’s a consequence of…” is the reason it exists if one does nothing, but one could consider to calibrate it out (partially) by design.

Not getting the point on using a flash… of course the accuracy of the shutter doesn’t matter in that case. I don’t even need a shutter. I can use my lens cap to open and close, as long as it’s dark enough to not expose.

Just arguing to make a silly point, I know. In the end, it doesn’t matter. I don’t think most folks take lots of pics at 1/500 or 1/250… and if you do, use an ND filter since most shutters are likely least accurate at the maximum speed anyway, and everything compounds.

[ic-racer]

One would have to go a long way back in history to find the source of this alleged engineering blunder.

Shutter Efficiency is the ratio of Te/To.

SHUTTERS, EFFICIENCY OF

The proportion between the length of the exposure and the time during which the lens is fully uncovered. Two shutters may give exposures of the same speed but of very different efficiency. Thus, one may take up the greater portion of the time in opening and shutting, while the other is fully open for the best part of the exposure. It thus results that with the first shutter a longer exposure is required to produce the same light effect as the second. -- Encyclopedia of Photography 1879

[ic-racer]

ISO 516 Shutter Speed Testing: Timing of the shutters should be measure at the appropriate aperture of the lens used. In evaluating shutters without lenses, exposure times should be measured under the conditions fixed so as to be equivalent to the requirements of this International Standard.

ISO 516 wants the shutter to be measured with 'Normal' lens wide open, as so:


(Front Shutter = Any shutter in the vicinity of the lens -- ISO 516)

But, if measuring only the shutter, one can get a near 'equivalent' measurement with the two measurements as shown here and using the equation:

Assume T1 = T3

Tt = T1 + T2 + T3
To = T2

Te = T2 + (T1 + T2)/2



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