Where do non-standard shutter speeds come from?

Question

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?

Answer 1

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   

Answer 2

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.