So Shrishti and I were out exploring one weekend and ended up at The Pearl, where she's practising shooting on manual. Take a shot... Way too dark. Adjust... Take another shot... Still much too dark...
So I ask, "What ISO are you on?"
"400", she replies.
I look up at the sky, look around where we are sitting, then look at what she's taking a photo of and say, "Try 1/200th of a second at f/8".
Oh snap! It's out by less than half a stop...
"How did you do that?"
"Oh, just experience..."
Yeah, I lied. It's actually experience plus a handy little rule commonly referred to as "the sunny 16 rule".
The sunny 16 rule says that around noon on a nice sunny day, the correct exposure is the reciprocal of the ISO at f/16. So with ISO 400 I would have said shutter of 1/400s and aperture of f/16 if she was shooting something in bright sun. But it wasn't bright sun - it was overcast, so I subtracted a stop for that (1/200s at f/16) - and it was about 4pm so I took off another half a stop - and the subject of the photo was a little dark and in the shade, so I subtracted another half-stop from that to arrive at 1/200s at f/8.
So I took the rule and adjusted it to fit the situation. The rule is the starting point, and knowing how much to shift it is experience.
Next time you're shooting outdoors, give the rule a try. f/16, and shutter of 1/ISO. Take the photo and see how far you have to adjust it to get a good exposure. Do it enough times and it will become second nature.
Now artificial light is a story that will have to wait for another day...