Exposure Quiz Question
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:
- 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.
- 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.
- 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.
- 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)