Quiz question: I want to shoot a night scene at f/16 ISO 100 with one stop of under-exposure. My camera's meter balances at f/8 ISO 400 with a shutter speed of 30 seconds. What shutter speed do I therefore need to shoot at my desired aperture and ISO?

Let's take the answer a step at a time:

1. The first thing I see is that I cannot balance my meter in any of the auto modes or even manual using my desired aperture of f/16 and ISO 100. This is because f/8 at ISO 400 is letting in a lot more light than my desired settings, balancing the meter for a normal exposure at 30 seconds. I'm going to need a slower shutter speed to let in more light, but the slowest automatic shutter speed available for most DSLR cameras is 30 seconds. I am therefore going to have to calculate the shutter speed and use Bulb shutter mode to hold open the shutter for the right amount of time.
2. The meter is balanced at ISO 400, but I want to shoot at ISO 100. So, let's calculate the shutter speed for ISO 100. ISO 200 is half as sensitive as ISO 400, which means I need to double the shutter speed to 60 seconds for ISO 200. ISO 100 is half as sensitive as ISO 200, which means for ISO 100 I need a shutter speed of 120 seconds.
3. Now let's make the adjustment for the aperture. We are balanced at f/8. Taking my ISO 100 adjusted shutter speed of 120 seconds, I now calculate the shutter speed for the smaller aperture of f/16. I know that f/11 is half the light (i.e. 1 stop) of f/8, so I need to double the shutter speed to 240 seconds. f/16 is half the light of f/11, so I need to double the shutter speed again to 480 seconds.
4. We have nearly done, but because I am shooting at night, my meter will tend to over-expose the scene, so I am going to under-expose by a stop. In really dark situations, I'd be closer to 2 stops of under-exposure, but never mind. One stop under-exposure requires me to use half the light of a normal exposure, so I simply half the shutter speed to 240 secs (4 minutes)

I'm ready to set my desired aperture and ISO and shoot the scene at 4 minutes shutter speed. I obviously need a tripod and will use a cable release to prevent camera shake while I hold open the shutter in Bulb. At this long exposure there could be some noise in the picture resulting from the camera sensor warming up while activated for such a long time. The advantage of digital photography is that it does not suffer from the same issues of exposure reciprocity faliure at long exposures that film does, making calculations like this straightforward with the added benefit of being able to review and adjust your exposures immediately. Digital is therefore a great medium for night photography and long exposure shots.