ISO - Exposure Question

[brettrdavies]

My question relates to ISO.

I have a Nikon D90 which has aperture in approximate half stop
decrements (4, 4.8, 5.6, 6.7, 8, 9.5). My understanding now is that as
the f number doubles say from f4 to f8 then I have moved 2 stops.
I also understand that as my shutter speed
which is also in half stop decrements (60, 90, 125, 180, 250, 350, 500
etc) doubles say from 1/125 to 1/250 then I have moved by 1 stop.

Now I also understand (think) that doubling the ISO, say from ISO 200
to ISO 400 moves 1 stop. However the ISO scale on my camera is 100,
130, 170, 200, 250, 320, 400, 500, 640, 800, 1000 which is pretty much
in 1/3 stops. Why is that? Given the aperture and shutter speed are
in half stop increments why is the ISO not so? How is that difference
handled?

For example; say my ISO is set at 200, shutter speed set at 1/500 and the
exposure is spot on with an aperture of f4.8. Then I want to
slightly increase the depth of field half a stop from f4.8 to f5.6.
Do I move 1/3 of an ISO stop to 250 or 2/3 of an ISO stop to 320 as I
don't have the option to move half an ISO stop from 200 to 300?

Am I missing something or is it just too pedantic. Any help would be appreciated.

[nickbedford]

Well I know that on my Canon 50D, I have the option of changing my ISO increments to 1/2 from the default 1/3rd. I imagine it's just a way of giving finer grain control over ISO exposure. The D90 may have this option.

[Sterling]

Custom setting B1 allows you to choose between 1/2 and 1/3 increments for shutter speed and aperture. 1/3 is the default setting.

[sk66]

If you leave the ISO in auto, as you change aperture the camera will select whatever ISO is right....even if it's an ISO setting/number you cannot select manually.

[OsmosisStudios]

Do yourself a favour: dont work in half stops but in thirds.