# Shutter speed series

Discussion in 'Casual Photo Conversations' started by david_lai|3, Oct 26, 2012.

1. ### david_lai|3

The number series for aperture size follows a logical sequence: 1, 1.4, 2, 2.8, ... etc., each number being multiplied by the square root of 2 to generate the next number. Makes sense, as the area of the aperture is being halved with each stop.

However, when it comes to shutter speeds, things aren't as logical. We have speeds doubled from 1 second to 1/2, 1/4, 1/8, but then it all goes wrong. Instead of 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024, 1/2048, 1/4096, .... we get 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, ...

In an industry in which accurate exposure is a major selling point of cameras, why is there this discrepancy between shutter speeds and aperture sizes? Come to that, as the sunny f-16 rule depends on matching film speed to shutter speed, why do films commonly get rated as ASA 25, 50, 100, 200, 400, 800, 1600, 3200,...?

2. ### alan_marcus|2

The shutter speed number set starts out OK but - the value after 1/8 should be 1/16 but it is internally set to 1/15 in the name of elegance. Error is so slight it is well within the precision the shutter can maintain. Now we go to 1/30 - 1/60 and next should be 1/120 but again in the name of elegance the next value in the set is set at 1/125. Again all the so called errors don't fall out of the needed precision which is 1/3 of a stop. The 1/3 stop rule-of-thumb takes in errors in the aperture setting and optical transmission.
You may disagree but it is generally impossible to set an exposure any closer than 1/3 f/stop. Additionally it is generally impossible to maintain the specifications of film processing any closer than 1/6 f/stop and most labs must apply due diligence to maintain even 1/3 f/stop as developing for a specific film speed.
The first to apply numbers to film speed was Julius Scheiner (German 1858-1913) who in 1894 established film speed numbers for astronomical glass plates. His method was a logarithmic notation just as star brightness scheduled by magnitude. This method called Scheiner was 1⁰ thru 20⁰ each value one hundred ford higher than the next.
The German system Dutshes Institut fur Normung (DIN) followed this method starting in 1934.
In North America Western Electric and General Electric marketed eclectic light meters and each had a different system and method to measure. The British also set up a system BSA and ASA in the United States.
The whole mess was unified by the International Standards Organization based in Geneva. This is the home of the ISO method of measuring film speed.

3. ### bebu_lamar

In an industry in which accurate exposure is a major selling point of cameras, why is there this discrepancy between shutter speeds and aperture sizes?

Huh? accurate? if the higher shutter speed is accurate within 10% you're lucky. Even with today technology the accuracy of the shutter speed isn't all that great. I do not like the way the shutter speed and even aperture number was round off. According to Nikon service manual the aim point for the the shutter speed are actually 1/16, 1/32 etc.. and not 1/15,1/30 although they are considered to be well within tolerance. You said aperture is ok? no f/5.6 should be f/5.7 but they want to round it off that way. Also ISI125 should be 126 to be a third stop faster than ISO100. But in photography that's the way it is. I just live with those number although it makes more difficult when one wants to docalculations. But I agree with Alan that the number are much more accurate than the obtainable accuracy. for example shutter speed 1/125 should be 1/128 but then one should be happy if his/her camera 1/125 shutter speed is actually 1/120 or 1/130.​

4. ### q.g._de_bakker

The technically correct series is indeed that sequence involving the powers of 2:<br>1 divided by 2^0, 1 over 2^1, 1 over 2^2, etc.<br>So the correct range is that 1/1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024, 1/2048, etc.<br><br>The differences between that and the nominal series is small. (You can calculate how small, in stops, by dividing 'should be' and 'nominal' and taking the log, base 2, of that quotient. The difference between for instance nominal 1/125 and 1/128 is 0.034 stops, between 1/15 and 1/16 is 0.093 stops.) Smaller than the inaccuracy of the shutters themselves.

5. ### john_robison|3

Ain't no mechanical shutter that accurate anyhow. Even ones with electronic control. Even if you select one particular speed, shoot indoors at 70F for a while then step out into 35F temps and shoot for a while. Probably will vary by 5~15%, even for the same speed. How accurate are the aperture settings? Doubt they are more 90% accurate. How about transmission through the lens, especially zooms, 15 elements in 11 groups, with 22 air to glass surfaces, what do you think that does to actual transmission of light through the lens? That is why lenses for movie cameras are marked in "T" stops, not F stops.
So I wouldn't worry about it.

6. ### david_klaffenbach

It is just one example (there are many) of a preferred number sequence (see http://en.wikipedia.org/wiki/Preferred_number), in this case apparently chosen for ease of use and manipulation while preserving more than adequate precision. You can trade stops of aperture for stops of shutter speed without worrying about the small error you might be leaving on the table. I'm not sure who first created these sequences (for aperture and shutter) but it arose during the era of slide rules when understanding of appropriate level of precision for a given problem was much more prevalent than today, when we can easily (and by default) generate as many digits of an answer as we care to take the time to read.
As an aside, I have my camera set on 1/2 stops rather than 1/3 stops for the aperture and shutter speed controls to make adjusting exposure faster and to expand the range shown by my spot meter. In principle, this means I might only be able to get within 1/4 stop of the "correct" exposure rather than 1/6 stop.

7. ### david_lai|3

Thanks for all the (very prompt) replies. I take the point that the inaccuracy of the number series is insignificant for practical purposes, but it's a mathematical thing, so why wouldn't they want to get it right? And I don't see how the correct sequence based on powers of 2 can be less elegant than some approximate sequence. It bothers me, but not so much that I'll lose sleep over it. Thanks to you all.

8. ### bebu_lamar

David I understand your feelings very well as it bothered me a lot too. It bothers me because the way they use the number that way and if I try to do a precise calculation the result don't match. So if one write the a program that deal with aperture and shutter speed one would have to use enumerate number and not using real calculation.

9. ### bob_sunley

Look at some older shutters for different sequences, like 1, 1/2, 1/5, 1/10, 1/25, 1/50, 1/100, 1/200, 1/400. Or maybe jump from 1/200 to 1/500, or 1/100 to 1/250. The number sequences were always off somewhere.

10. ### lex_jenkins

Reminds me of that Northern Exposure episode "Nothing's Perfect" where Maurice complains that his expensive ornate German clock doesn't also keep perfect time.
Every digital camera I've owned or tried was remarkably accurate, within 1/3 EV at every setting. Whenever I noticed more than a 1/3 EV deviation the inaccuracy was with the lens - particularly with older lenses used on dSLRs.
With film era cameras, even when new, we'd routinely run some test rolls to see whether we'd need to make adjustments to accommodate deviations from nominal specs. It's not unusual to find film cameras with 1/2 EV deviations at shutter speeds slower than 1/15th second or faster than 1/250th.

11. The aperture series is rounded as well. The square root of 2 is 1.4142135623730950488016887242097 and f/5.6 is really 5.6568542494923801952067548968388 not f5.7 !