Why are 1/60 and 1/125 considered one full stop apart?
Asked 8/15/2017
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I’m confused about shutter-speed full stops. If halving 1/60 second should give 1/120 second, why is the next standard full-stop setting 1/125? Does 1/125 really let in half as much light as 1/60?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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The 1/60 or 1/125 are just nominal approximations, not actually existing. The actual values used are 1/64 and 1/128 second, in the binary progression of 1, 2, 4, 8, 16, 32, 64, 128, 256, 512. etc (each exactly double the previous). Those were strange numbers 100 years ago, and the nominal numbers are still shown (by convention today), thought to be easier for humans to handle. But the camera knows to instead do it right. See my site http://www.scantips.com/lights/fstop2.html
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
8y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—1/60 and 1/125 are treated as one full stop apart because the displayed shutter speeds are rounded standard values.
A one-stop change means doubling or halving exposure time. The exact binary sequence would be closer to: 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256, etc.
In that exact progression, half of 1/64 is 1/128. Cameras label these as the more familiar rounded numbers 1/60 and 1/125. So:
- 1/60 is a rounded display for about 1/64
- 1/125 is a rounded display for about 1/128
That means they are effectively one stop apart, and 1/125 does give about half the exposure time of 1/60.
The same kind of rounding happens elsewhere in the shutter-speed scale, such as 1/15 instead of 1/16 and 1/30 instead of 1/32. So the numbers you see are conventional approximations, while the stop spacing is what matters.
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