I used to shoot with primes and this made using Auto ISO so very simple with my then preferred Aperture Priority. The the minimum shutter speed and maximum ISO and get on with it.
Now, I've become fond of the zoom lenses but their varying focal length makes Auto ISO a bit problematic with regard to selecting a minimum shutter speed. I don't care for Shutter Priority much with Auto ISO as the camera will then stop the lens down to an aperture I really don't want to use. And if I need to stop action I can use an appropriate shutter speed without compromising the particular aperture effect I want to get.
But.... in Manual mode I can select the aperture I want to use at that given time as well as pick the shutter speed that will give me a good handheld capture relative to the focal length to which I have my lens set. Since I'm changing shutter speeds much more than aperture settings, the D40 just shines as the control wheel is dedicated to shutter speed in manual mode and requires no other button activation. Very cool, Nikon - thanks! Now I have many more shutter speeds available as compared to the Auto ISO selections.
Why Auto ISO? Because this forces the D40 to give me all those lovely intermittent ISO steps between the normal settings found in the menu selections. Why jump from ISO 400 to ISO 800 when I can have a few steps in between?
I let Auto ISO run to 1600 if needed, after all, I want to get the picture so if the choice is between doing so with a bit of noise or not getting the capture, I'll take the image, thank you! You can't clean up a capture you didn't get Besides, this is the D40 we're talking about.... noise just ain't a big issue.
Give it a whirl and see what you think. If you're coming in from the P&S world, switch that D40 out of P mode and force it into submission! Make it behave as the wonderfully versatile little computerised beast that it is.
I discovered something strange with my D40 when I tried to set it to auto ISO. If I have the camera in A, S, or M and go the ISO selection menu and try to select "Auto" I get an error message that the option isn't available with the current settings and it defaults to 200. The LCD shows the ISO as 200, but on the left side is a flashing "ISO-A" and it does actually work. I took a picture in very low light and the ISO was 1600 according to the EXIF data. Is it just my camera or have you experienced that also?
This is my most common way of working when I want to specify both the shutter speed and aperture, but also want auto-exposure. Auto-ISO is a great way to get auto exposure in manual mode.
I most commonly shoot this way with spot metering when I'm chasing birds in flight, and also when I'm shooting something that's moving quickly through different lighting where I know I want a certain amount of motion blur, like a propellor plane.
I was really curious about the minimum ISO when the camera is set to Auto-ISO so I did a little experimenting today. It was partly cloudy today so I set the lens to f/11 and the shutter to 1/1000. I took successive pictures starting at 1/1000 and halving the shutterspeed each time. I was hoping that the ISO would go below 200, but no such luck. It hit some intermediate ISOs but I got 4 consecutive shots with increasingly blown highlights all at ISO 200.