Where do non-standard shutter speeds come from?

When I google "shutter speed" and explore a few first hits, there is always listed only the standard shutter speeds - 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15 etc

But, when I go looking at photos in Flickr, I sometimes see photos taken with shutter speeds like 1/320 or 1/80 and similar oddities.

Where do these odd speeds come from?

I know we are talking about electronic devices here, so naturally we are not tied to the old mechanical camera limitations. So, can a shutter speed 1/320 be manually chosen in a modern digicamera, or is it a product of camera set to Auto-mode?

-
Among all the answers is a tiny detail that may make this easier to understand. There is a difference between the setting indication and the actual shutter speed. You can set the shutter speed to indicate your desire; but, in actuality, the shutter speed will not occur as indicated. – Stan Aug 31 '13 at 22:50

Those listed are full stops. Most cameras allow you to increment shutter speed and aperture in half-stops or one-third stops, and you can select intermediate values manually.

• If you have the camera set to half-stops, then you'll have 1/350 between 1/250 and 1/500.
• If you have 1/3 stop increments set, you'll have 1/320 and 1/400

To work these out, a full stop is double the light. A half stop then is the square root of 2 times, or 1.4 (so that if you go up a half stop, then another half stop, you multiply the 1.4 factor together, and 1.4 * 1.4 = 2, which is your full stop)

• So 250 times 1.4 = 350
• and 350 * 1.4 = 500

For 1/3 stops, it's the cube root of 2, or 1.26x

• 250 * 1.26 = 315 (rounded to 320)
• 315 * 1.26 = 396 (rounded to 400)
• 396 * 1.26 = 500

Note that numbers are rounded, considerably in some cases, for convenience. The actual shutter speeds the camera produces are probably more precise values than these.

1/2       1/3
Stops    Stops

10001000
750 800
500 640
350 500
250 400
180 320
125 250
90  200
60  160
45  125
30  100
23  80
15  60
11  50
8   40
6   30
4   25
3   20
2   15
1.5 13
1   10
8
6
5
4
3
2.5
2
1.6
1.3
1

-
And yes, some cameras have stepless shutter speeds in P, A/Av, full auto and scene modes. – Ryccardo Apr 1 '13 at 9:48
@Ryccardo Which cameras? – mattdm Apr 1 '13 at 17:32
I personally know the Nikon W35 and the Electro Yashica 35 do :) Even many digital cameras do -- however you'll only see that in the EXIF. See photo.net/nikon-camera-forum/00LgqA – Ryccardo Apr 1 '13 at 18:57
1/250 sec is really 1/256 sec. Every thing is based on powers of 2 starting with 1 second (2^0). The 1/2 stops steps above 1/250 (256) are 362.04 and 512. The 1/3 stop steps are 322.54, 406.37, and 512. – Michael Clark Apr 1 '13 at 22:01

The list you quoted are full stops. Most cameras will also allow you to select 1/2 or 1/3 stops, which are spaced in between the full stops. Just as with f/numbers, they are often rounded to make them easier to remember and refer to. Since they are mathematically based on √2 and ³√2 they don't always compute to nice, even numbers.

Depending on what application you are viewing your photo with, sometimes both the shutter speed (Tv) and aperture (Av) setting and the actual Tv and Av will be displayed in the EXIF information. For example, I just looked at a photo using Irfanview that was manually exposed at 1/80 sec. @ f/3.5. In the EXIF information Exposure Time was displayed as 1/80 seconds and FNumber was displayed as 3.50. Further down in the EXIF info Shutter Speed Value was displayed as 1/83 seconds and Aperture Value was listed as F 3.51. These numbers are not the theoretical mathematical values expected, but rather the value that my camera/lens actually uses when a picture is exposed using those settings.

-