[keith_d]

Sorry, this is going to be a bit academic....

The odd values for shutter speed all start because the amount of light collected by a lens depends on the area of the lens and it's focal length. It's rather tricky to measure the area of a lens, but it's fairly easy to measure the diameter of an opening. This led to photographers used something called the f-stop.

f-stop = aperture (diameter) / focal length

So, f/4 indicates that the effective aperture is a quarter of the focal length, and f/8 means that the aperture is one eighth of the focal length. Photographers soon discovered that changing the aperture by 41% doubled or halved the amount of light reaching their film. So we end up with a series of f-stop values like this:

f/2.8
f/4
f/5.6
f/8
f/11

Each of them halving the amount of light reaching the film/sensor.

To avoid masses of tedious calculations we use a similar scale for the shutter speed. On old cameras one step on the shutter dial either doubled or halved the time the shutter was open. So you would see speeds like 1/60, 1/125, 1/250, 1/500 etc. representing the fraction of a second the shutter was open. Once you had taken a lightmeter reading you could select from several sets of values, each of which would expose the film to the same amount of light. For example all of the following would give the same exposure.

1/60 at f/16 (less light from lens, shutter open for longer)
1/125 at f/11
1/250 at f/8
1/500 at f/5.6 (more light passing through lens, shutter open for less time)

Modern cameras can be more precise and have several steps in between each of these shutter speeds. On your camera each of these steps is split into three smaller steps (one third of a stop). Since they follow the same rule as the larger steps, each must increase the shutter speed by 26%.

1.26 * 1.26 * 1.26 = 2.00

This leads to values like this:

1/250
1/320 (250 * 1.26 rounded)
1/400 (320 * 1.26 rounded)
1/500

This is all a bit academic when it comes to taking pictures. So I wouldn't worry too much about the rather odd numbers.

Keith.