Why do standard shutter speeds use values like 1/125 instead of exact halves such as 1/120?
Asked 4/27/2014
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Standard full-stop shutter speeds are usually listed as 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and so on. Since each full stop should halve the amount of light, why isn’t 1/125 written as 1/120, which is exactly half of 1/60? Is this just arbitrary rounding, or is there a historical or technical reason behind the standard shutter-speed sequence?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Actually 1/125 is half of 1/60, ±0.06 f-stop.
It should be obvious by looking at shutter speeds that they were chosen to be the reciprocal of nice round numbers. Start with 1 second and keep dividing it by 2. Note that you missed the discrepancy between 1/16 s and 1/15 s. If you kept going in strict mathematical multiples of 2, then 1/60 s should really be 1/64 s, 1/1000 s should really be 1/1024 s, etc.
The basic problem is that in photography we are used to dealing with factors of 2, but a sequence of factors of 2 doesn't work out to nice numbers in our decimal numbering system. So we observe that 103 is close to 210, and realize that the 0.034 f-stop error is inconsequential.
Adding slight shifts in the factor of 2 progression to the shutter speed sequence in order to keep them as round numbers in our decimal system allows people to do mental math on them more easily.
Originally by user7603. Source · Licensed CC BY-SA 4.0
user7603
12y ago
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Yes—there is a sane reason: standard shutter speeds are rounded conventions, not exact powers of two.
A full stop in shutter speed halves or doubles exposure, so the exact sequence would be 1/16, 1/32, 1/64, 1/128, 1/256, 1/512, 1/1024, etc. But photographers and manufacturers preferred easy decimal-friendly numbers, so these became 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, and so on.
So 1/125 is the rounded stand-in for 1/128, just as 1/60 stands in for 1/64 and 1/1000 for 1/1024. The exposure difference is tiny—small enough to be insignificant in practical use and often smaller than the real-world precision limits of shutters, especially in older mechanical cameras.
There’s also historical convention involved: shutter-speed markings were standardized around practical, familiar numbers rather than mathematically exact ones. The same kind of rounding happens with f-stops too, such as f/1.4 and f/2.8.
So the sequence isn’t broken—it’s just rounded for usability.
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