I have set ISO=200 as the base and 3200 as the max. When I shoot at A-priority f2.8 under room lighting condition, with a minimum shutter speed of 1/80, the CPU selects 1/100 sec exposure and the ISO remains at 200. Apparently, the priority for a 24-70 mm lens is to try to keep the ISO as low as possible and change the shutter speed for proper exposure.
What if I wanted the CPU to increase the ISO to 3200 and increase the speed to 1/1280, for example. Would it be necessary to increase the minimum shutter speed to accomplish this? Of course, I could just shoot manual...
#3. "RE: Logic of D700 CPU" In response to Reply # 1
>The Minimum Shutter Speed setting in ISO Auto means the >slowest speed that will be used before the camera raises the >ISO. > >1/100th is faster than 1/80th, so the camera is doing >just what you told it to
Yes, I realize that. The CPU has a choice at A-priority in low light: do I use a slower shutter speed and keep the ISO the same or do I boost ISO to allow a faster shutter speed--or some combination of both? As you pointed out, there is a minimum shutter speed setting that can be used to ensure a fast shutter speed and allow the ISO to increase to ensure proper exposure. That's the solution, but the minimum shutter speed would have to be changed depending on the focal length of the lens.
Getting back to the question about CPU priority...let's say I set the minimum shutter speed at 1/30 in ISO Auto and a max ISO of 6400 and use a 200mm lens. Furthermore, a proper exposure is 1/30 at ISO 200. Does the CPU read the focal length and increase the shutter speed to at least 1/200 even if it means increasing the ISO to compensate or does it keep the shutter speed at 1/30 to favor a lower ISO? Which has priority: ISO or shutter speed?
If the CPU was smart, unlike the photographer, it should ensure there is a minimum shutter speed based on the focal length of the lens--but then it most likely wouldn't know that the whole system might be on a tripod where 1/30 is acceptable for a 200mm lens and an ISO of 200 is desired to minimize noise.