I have never used Auto ISO mode on the D200 because I typically want to know all of the settings (shutter-speed/f-stop/ISO) when I take a picture. However, I am taking more action shots in changing light conditions where Auto ISO may be useful. What I don't understand is the prioritization of the ISO change. For example, in (S) mode, if I choose a shutter speed of 1/500 and am using my 17-55 2.8 lens, will it use the lowest ISO (i.e., 100) and reduce the dof all the way down to 2.8, or will it choose a more optimal dof for the lens (e.g., f8) and increase the ISO accordingly. Then, say it selects ISO 200 and f4 for one shot, will the next picture default to ISO 200 or will it again try to optimize for ISO100? You can see I am a bit confused, and the manual doesn't help much. I just want to know whether shooting Auto ISO means that you tend to be at the fringes of your dof if you are shooting in (S) mode.
I rarely use Auto ISO, but when I do, I use it in M mode. I set the shutter speed according to the action stopping and/or handholding requirements and the aperture according to the DOF requirements, and let the camera satisfy those conditions with auto ISO.
To answer your question, the camera always tries to optimize the ISO as low as possible.
I have not tried to decipher the logic (fuzzy? ) behind auto ISO in A or S modes. And using Auto ISO with flash can be even more confusing.
Analyze your shooting requirements and decide, whether M mode with Auto ISO is adequate.
Pete "Cameras don't take photographs, people do" - John Hedgecoe said that. "Expose for the highlights and let the shadows take care of themselves" - Ansel Adams said that. "The camera is only a tool. The best saw in the world won't make you a great carpenter" - I said that A few photos, here for a reason
In "S" mode, first opens up to max. aperture, then changes iso. In "A" mode, tries to use the set shutter speed (and not the lowest, i.e. 30s), then changes iso. This is somehow inconsistend. There should be an option to set a preferred aperture in "S" mode, but it isn't. It's always the max. aperture of the lens used.
When using flash, things are more complicated, and I don't know, what's going on then. Now there are three variable parameters: Shutter speed or aperture, iso sensitivity, and flash output power. I don't know, how this is handled. Manybe someone could tell me?