View Full Version : Shutter speeds

Charles Forde
14-Dec-2004, 05:41
Why do shutter speeds go from 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000 per second.
Shouldn't it go 1, 2, 4, 8, 16, 32, 64, 128, etc.

Matt Miller
14-Dec-2004, 06:09
Mine go 1, 2, 5, 10, 25, 50

Emmanuel BIGLER
14-Dec-2004, 07:07
Charles. Your are perfectly right. Actually the tolerance allowed for "mechanical" shutter speeds is not very stringent, about 10% is acceptable. Between 128 and 125 the relative difference is smaller that 3%, it is a negligible error. So you can consider in fact that 1/125 = 1/128, 1/500 = 1/512, etc.. The actually engraved series of numbers is purely conventional. Some modern 35 mm SLR cameras with a built-in electronic shutter timing unit actually generate the 32 - 64 - 128 -256 series simply because the timing is generated by a cascade of binary frequency dividers, most probably starting from a wristwatch crystal oscillator @ 32768 Hz ; 15 stages of binary dividers will deliver all shutter speeds you need down to 1 second.
Matt. the series you mention is the 'old German' one. It was in use until the 1960's, I have a compur#2 of the sixties engraved like this, it was eventually replaced by Charles' "modern" series, which should actually be labelled : 32 - 64 - 128 - 256.. to be really "modern" ;-);-)

Charles Forde
14-Dec-2004, 09:49
Thanks for the replies, what were the reasons for the 'odd' shutter speeds, was it easier to make? Not knowing how a mechanical shutter works, I would of thought it would be like a gear box. Just changing ratio's 1:1, 1:2, 1:4 etc. or would that of been two bulky.
Also have polarisers ever been used a shutter ie. using two polarisers or would there be too greater a reduction in light. I would have thought it would be handy for fast flash sync.

Ralph Barker
14-Dec-2004, 10:10
We didn't have shutters back when we were doing portraits of wooly rhinos in the caves. Instead, we had "shudders" - usually right after the rhino noticed us "taking their picture" without a paid permit. ;-)

Although I have no documentary support, I suspect the shutter speeds were derived from a desire to generate "conventional-looking" fractions, working backward from the fastest speed mechanically practical to obtain.

Alan Davenport
14-Dec-2004, 10:17
This is a WAG, but in view of human proclivities it seems logical to me: <B>you're approaching the sequence from the wrong end.<B>

Let's say they started at the fastest speeds their technology could create, say 1/500. From there they began halving the denominator to get slower speeds, 1/250 and 1/125.

The next step in that sequence gives 1/62.5, but this is not a number humans like to deal with. We much prefer to work in whole numbers, and in fact 1/60 is much more acceptable than 1/62 or 1/63, so the next step became 1/60.

That was followed, of course, by 1/30 and 1/15, but again we have a problem at 1/7.5, so 1/8 it was. From there things stay simple, logical and, more importantly, the numbers that don't make people's eyes glaze over.

Let's keep in mind, the difference in exposure with these time changes is trifling, so small that it obviously doesn't even make a difference to our limited-latitude color transparency films today. Much less did it matter to wide-latitude, thick film B&W films that were in use at the time.

Alan Davenport
14-Dec-2004, 10:17
Sorry about the bold. Clicked the wrong button.

Charles Forde
14-Dec-2004, 11:48
Thanks, I was approaching the sequence from the wrong end.

Philippe Gauthier
14-Dec-2004, 15:56
</b>Thanks for the explanation, Alan. And I hope this message will effrectively rid us of the bold character set.

Alan Davenport
14-Dec-2004, 16:21
Still bold. </B> Lemme try again.

Emmanuel BIGLER
15-Dec-2004, 04:37
I recommend this excellent web-document by Rick Oleson, explainig how a Synchro-Compur shutter works. http://rick_oleson.tripod.com/index-88.html (http://rick_oleson.tripod.com/index-88.html)