D7i AUTO ISO feature
Many people avoid the Auto ISO feature on the D7 and D7i for fear that the camera will jack the ISO up and introduce noise without their knowing it. While this is a legitimate concern, the Auto ISO feature is actually very conservative and probably does something very close to what you would do anway; see below. I've conducted a series of tests to find out how it works.
In a nutshell, the way that the Auto ISO feature works is this...
The camera will attempt to use ISO 100 above a certain shutter speed that I am calling the "magic" shutter speed. When the shutter speed would fall below the "magic" speed, the camera will increase the ISO to try to hold the shutter speed. There are two "magic" speed for the ISO boost to occur, the ISO can go up to 200 at the first one, and up to 400 at the second one.
So, what are the magic speeds? The actual speed at which the ISO boosts occur varies with the focal length, just as it should. The camera uses a higher magic speed at longer focal lengths to reduce shake. In fact, the first magic speed appears to almost follow the old 1/focal length rule for hand held shots. So, for example at 200mm the first magic speed is about 1/200 seconds. At 50mm the magic speed appears to be 1/45 seconds. The exception to the rule is that at 28mm the magic speed remains 1/45 second.
As I mentioned there are two magic speeds. The camera will not use ISO over 200 until the speed falls lower than the second magic speed. This appears to be about 3-4 seconds, so you will not usually have to worry about it.
Yes, the camera can use intermediate ISO values, but only right at the magic speed where the camera is trying to hold the shutter speed up. So, for example, at 28mm you might see ISO 125 or ISO 160, but only at 1/45 second where the camera is trying to hold the shutter speed. Once the shutter speed drops below 1/45 second, the camera will be using ISO 200 all the time.
So, the camera does what you might do in most handheld situations anyway. It only uses ISO above 100 when it has to to hold up the old 1/focal length rule for handheld shots, and then does not go above 200 until you get out to 3 or 4 seconds. That is pretty concervative. Which is worse, a blurry shot from "shake" or an ISO 200 shot; it is hard to decide.
Here is an example at 28mm. As the light falls the camera might use
1/500 at smaller apertures, ISO 100
1/250 at smaller apertures, ISO 100
1/200 at smaller apertures, ISO 100
1/200 at larger apertures, ISO 100
1/200 at largest aperture, ISO 100
1/200 at largest aperture, ISO 160 (trying to hold first magic speed)
1/200 at largest aperture, ISO 200
1/180 at largest aperture, ISO 200
1/125 at largest aperture, ISO 200
1 second at largest aperture, ISO 200
3 seconds at largest aperture, ISO 200
4 second at largest aperture, ISO 200
4 seconds at largest aperture, ISO 300 (trying to hold second magic speed)
4 seconds at largest aperture ISO 400