Auto ISO Query . . .
I shot the following image on my D7000 with my NIKON 80-200 f/2.8 AFS on Aperture Priority f/4 with . . . AUTO ISO ON . . . MAX SENSITIVITY 6400 . . . MINIMUM SHUTTER SPEED 1/800 . . . I think I spot metered on the white of the nearest boat . . . and the camera selected an exposure of f/4 . . . 1/4000s . . . ISO 640.
The camera was hand held while I was standing on a motionless but vibrating ferry preparing for departure.
The NEF file has been processed in DxO Optics Pro with default sharpening and straightened and cropped to about 70% of the full original image then opened in ACR with a little further exposure adjustment but no sharpening then saved as a JPEG at Quality 10.
Image is best viewed as original then click on 100% . . . note the sharpness of the lettering on the hull of the nearest boat . . . approximately the focus point.
So why did the camera select 1/4000s at ISO 640 . . . why didn't it select say 1/2000s at ISO 320 . . . or 1/1000s at ISO 160???
I'm actually glad it did select 1/4000s because I got about as sharp an image as I could get short of using a tripod.
If I had shot in manual I probably would have shot at about 1/1000s and the image would not be as sharp although the quality might be slightly higher at a lower ISO.
But I also would have thought the camera would have chosen values closer to the minimum set shutter speed [1/800s] with a lower or lowest ISO value.
So . . . how does the camera make the selection???
Does it know what shutter speed ISO combo to select to achieve optimum sharpness / quality . . . or is there some other criteria it processes to arrive at the selected exposure?
Any enlightenment here would be greatly appreciated!