the halving of 1/60 is 1/120 then why is that standard 1/125 for a full stop? is 1/125 really cutting the amount in half?


  • ...I read the answers of the dup question. The answer provided here is better.
    – Tampa
    Aug 16 '17 at 0:10
  • 1
    Then the proper way to do it here at stack exchange is to submit that answer to the existing question.
    – Michael C
    Aug 17 '17 at 8:06

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

  • I don't think it's that old because it wasn't until the 50s when shutter has even steps. Aug 15 '17 at 14:56
  • Maybe the exact year border lines are a bit debatable, but even in the 1910s, many camera shutters had steps, often T, B, 1/100, 1/50, 1/25 second. For one example, see licm.org.uk/livingImage/Butchers_Carbine.html from 1915. Yes, later 1930s and 1940s numbers were more like todays numbers, so maybe 85 years (but faster than 1/500 or slower than 1 second were still unsolvable mechanical problems). Yes, the old mechanical springs and gears certainly made it very difficult, as compared to todays digital timing chips making 2x steps be easy to implement (when cameras had batteries).
    – WayneF
    Aug 15 '17 at 16:53

As WayneF has told you, the shutter speed sequence is 1 - 1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500. These are fraction of a second. Consider that this sequence should be as WayneF has written, however for elegance sake, some values have been rounded. As an example 1/16 is rounded to 1/15 and 1/32 is rounded to 1/30 etc.

These values are fractions and many people have difficulty with fractions. 1/60 can be re-written as 0.016 of a second and 1/125 can be re-written as 0.008. These are decimal fractions. It would not do you any harm to brush up on fraction math. Note the difference in time between 0.016 and 0.008 is a 2X factor or a doubling or halving. Also you need to know that the f-number system is based on a doubling or halving sequence. We are talking about how much light is allowed to play on film or digital chip. A 2X change was deemed OK for most photography.

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