Originally posted by Duck Dodgers It does increase the sensitivity, but not in the way you describe. When you run out of aperture and cannot increase the shutter speed, if the scene darkens any from that point on, ISO increases, holding that shutter speed and aperture. (I just tried it using the light from my monitor; as I got to 1/160 at f/1.7, I moved the camera view away from the monitor, and ISO increased from 100 to 800, which is the Auto-ISO range I have set.)
You do that without adjusting shutter speed. I am talking about situation when you adjust shutter speed in P.
Do following.
Set camera to Auto ISO.
Put camera on tripod or any other stable place, so light coming to camera will be constant.
Set camera to Tv mode, shange shutter speed both ways and look how Auto ISO is behaving.
You will notice that Auto ISO algorithm is straightforward: when you increase shutter speed, ISO is increased as the last resort, when lens maximum aperture is reached and lens can't be opened more. When you decrerase shutter speed, ISO is lowered firs and when lowest allowed ISO value is achieved, camera starts to close aperture.
Then switch to P mode and start to adjust shutter speed and look how it behaves. According to manual and as advertised, in P mode when you turn front dial, camera seamlessly switches to Tv mode.
You will see that camera does not behave like it was behaving in Tv mode. It will lower ISO, but it won't increase it.
Most interesting situation you get in low light.
For example your lens is f/2.8 and in P mode camera meters 1/60, f/2.8, ISO400. With front e-dial decrease shutter speed to 1/30. aperture will stay at f/2.8, ISO will become 200. Then try to go back to initial shutter speed 1/60s. Camera won't allow this. And if you decrease shutter speed to 1/15s, settings will become f/2.8, 1/15s, ISO 100. Camera then won't allow you to select anything faster than 1/15s. Finally if you would press green button, camera will set 1/60s, f/2.8, ISO 400...
For me it's clearly a bug. And they fixed it in K-7. K-7 P mode acts exactly the way I expect it to act.