Why are you shooting at f/16 and playing with ISO? I would do the opposite: leave the ISO as low as possible (best image quality) and only adjust the aperture/shutter speeds. ISO is the LAST thing I play with.
I pretty much only use Full Manual (M). It gives me all the control over the image. Yes, it takes more time, but it gives me that much more instant control. When time is an issue (photojournalism, mainly) I switch over the P sometimes, depending on the conditions.
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