The discussion of Aperture versus Shuttler Speed versus ISO are the fundamental basics for understanding photography. A thorough understanding of the relationship of all three are necessary to understanding exposure in digital SLR cameras.
This guide provides a simplified explanation of these terms and their effect on each other.
What is aperture?
Aperture is the relative size of how wide the lens opens when the shutter is released. It is one of the factors that determines how much light enters the camera. The other factor is shutter speed, which determines how long the shutter stays open.
Here is the f-stop scale
Each step on the scale lets in half as much light as the previous step. So f/1.4 lets in half as much light as f/1, f/2 lets in half as much light as f/1.4 and so on.
You can remember the f-stop scale by remembering that the first two numbers are 1 and 1.4. All the others are doubling of those two numbers.
How does aperture relate to shutter speed?
Well, suppose you have a correct exposure at f/4 and 1/100 sec. If you decreased your aperture to f/5.6 what shutter speed would you need to get the same exposure? f/5.6 lets in half as much light as f/4. That means you have to leave the shutter open twice as long to get the same exposure. So at f/5.6 you would need to set the shutter speed to 1/50 sec to get the same exposure.
Suppose on the other hand you want to go from f/4, 1/100s to 1/200 sec. What f-stop would you need? 1/200 sec is half the exposure of 1/100 sec. That means you have to let in twice as much light to get the same exposure. So at 1/200sec you have to increase your aperture to f/2.8
Why would you choose one shutter speed over another, or one f/stop over another?
Suppose you’re shooting a moving subject. If you want to freeze the action you’d choose a fast shutter speed, but you’d have to open up the aperture to let in more light. However, a wider aperture also leads to a shallower depth of field. You’ve probably seen sports photos of athletes frozen in mid air with the the background out of focus. Fast shutter speed and wide aperture will do that.
If you want to increase your depth of field you’d choose a smaller aperture and a slower shutter speed.
How do you know when you have the right combination of aperture and shutter speed?
- You could take a test shot, review and adjust. Look at the histogram:
- if the histogram is too far to the left it means your exposure is too dark. Open up the aperture or use a slower shutter speed.
- if the histogram is too far to the right it means you exposure is too bright. Close down the aperture or use a faster shutter speed.
- You could use the camera’s built-in light meter. Your camera will tell you when it thinks you have the right exposure. See your camera’s user manual for instructions on how to use the light meter.
How does ISO relate to all this?
ISO sets your camera’s sensitivity to light. ISO 200 is twice as sensitive to light as ISO 100; ISO 400 is twice as sensitive to light as ISO 200 and so on.
Suppose you have a proper exposure at ISO 100, f/4.0 and 1/100sec. If you set the camera to ISO 200 you can set the camera to f/5.6 & 1/100 sec, or f/4.0 and 1/200 sec an get the same exposure.
Why would you change the ISO setting?
Suppose you’re in a low light situation. You’ve got the lens at it’s widest setting and the shutter speed as slow as you’d dare take it (any slower and the picture will turn out blury because of movement, etc), but your exposure is still too dark. You can boost the ISO to get a better exposure.
Be warned, higher ISO means lower saturation and more noise.