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