BINGO! Got it. I'm going to save that because it's as good a way of explaining it as I've seen.
One way I think about it when I'm out there is:
- Okay, I want to be at ISO100.
- What aperture do I want for this shot (and that is about DoF)?
- Okay, with those two set, what shutter speed does the camera tell me I need? Is that going to be okay, or do I need to adjust one of the other two variables in order to be able to get a different shutter speed?
Now, to go back to your example above. If someone told you to move from 1/250th to 1/160th to get more ambient light (without changing aperture or ISO setting), then that is right. You're slowing down the shutter but still goign with teh same aperture value. So, more light is going to get in. That will give a brighter photograph. But that then is where the Histogram comes in to play, because if you keep doing that, then at some point (1/160th, 1/125th, 1/90th) you're going to be over-exposed. The histogram and the 'blinkies' (have you got tot hem yet?) are there to help you with that management of the exposure.