 BetterPhoto Member |
History, standards on shutter speeds Well, I'm in a photography class, and we were learning about shutter speeds a while back, and what I wondered is why the speeds change, like so: 2s, 1s, 2, 4, 8, *15*, 30, 60, *125*, 250, 500, 1000. Where instead of 16 there is 15, and instead of 120 there is 125? I'm wondering the science/history behind that, and why it is the way it is.
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BetterPhotoJim.com - Jim Miotke  Contact Jim Miotke Jim Miotke's Gallery |
This is a fun question, Jason. Perhaps some brilliant historian will drift by and suggest an answer for us. In the meantime, I've asked the curator of Eastman House and this is what he said:
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Ho Kheong Fong |
I am a mathematician and I am trying to answer your question based on intuition and logic. If you look at the numbers carefully, whenever the number is smaller than 10 it just doubles without any fuss. However, when the number is more than 10 then some numbers do not follow the pattern. The reason could be because it is difficult to use numbers which are difficult to remember such as 16 when you double 8. As you go one doubling, you get 32, 64, 128, 256 and so on and they are double or triple digits which are difficult to remember. So the intelligent guy decides to use numbers which are easy to remember such as doubling 15 you get 30, then 60 and then suddenly change to 125 as doubling 125 is 250 and then 500 and 1000 which are easier to remember. My answer is hypothetical based on mathematical logic and my own intuition.
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Dick |
It looks to me like the manufacturers chose to mix two different standards for shutter speeds. The early British shutter speeds above 1/10th sec were based around 50, i.e. 1/25; 1/50; 1/100; etc. However, North American camera makers chose speeds based around 60. I have the feeling this was somehow connected to the fact that the electrical mains frequencies were 50 cycles per sec in England and 60 cycles per sec in the US. As the electrical power providers keep those frequencies very accurate over time, they are a useful standard against which to calibrate shutter speeds with relatively simple tools (probably a neon lamp stroboscope was used.) But like the non-uniformity of aperture progressions ( 2,8; 3.5; 4.0; 4.5; 5.6; etc, it could just be whimsy on the part of the designer. In the end, it doesn't matter what the numbers are as long as you know what to expect when you use them!
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John A. Lind |
Technically, it is all related to a one second exposure and each "stop" of shutter speed either doubles or halves the amount of light admitted to the film which I think you intuitively know already. Therefore, if you followed it strictly it should proceed to 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024, 1/2048 and 1/4096, the latter two being found on newer and high-end camera bodies. In the grand scheme of things, the difference between 1/64 and 1/60 is so slight that it doesn't matter. You would have to be 1/3 stop off or more to notice a difference on slide (reversal) film which is the most sensitive to exposure error. At 1/1000, a third stop slower would be 1/800 and a third stop faster 1/1250. The rounding is much easier to remember and even the most expensive 35mm camera shutters are not accurate enough to distinguish between 1/128 and 1/125. You might be surprised at how far off many cameras really are and the user will never know the difference because it is not detectable in the photographs. If your camera really opens the shutter for 1/940 when you set it to 1/1000 and you can't notice it in the photographs, does it really matter? On much older cameras, the slower shutter speeds were rounded even more. My 1950's Zeiss Ikon Contax rangefinder follows the scheme from 2 seconds to 1/2 as before, then goes to 1/5, 1/10, 1/25, 1/50 and 1/100 before switching back to 1/250, 1/500 and then leaping 1-1/3 stop to 1/1250 (the last one being done to claim fastest shutter speed on the market which it was at the time). All are within 1/3 stop of halving the shutter speed and from a practical standpoint don't make a difference with negative film. I do watch the ones that are 1/3 stop away from exactly half and will adjust the aperture slightly, but only when using slide film. Aperture f-stop numbers are rounded off even more than shutter speeds and date to the 1800's when the openings were physically measured with a ruler. -- John
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