Exposure is one of the keys to good photography. Good exposure depends on three controls: shutter speed, aperture and ISO. Collectively they’re known as the exposure triangle.
Working together, these three settings get you to a photo that’s neither too bright or too dark. Shutter speed and aperture control how much light gets into your camera, and ISO controls how sensitive your camera is to light.
Steps (sometimes called stops) on all three of these settings are measured in Exposure Value.
In the example at the right, we’re getting a good exposure with a shutter speed of 1/15 of a second. Slowing the shutter down to 1/8 of a second lets more light into the camera and makes the image one EV brighter. Once we get all the way to +4 EV, the shutter is open for a full second and the image is way too bright.
Moving in the opposite direction, speeding up the shutter to 1/30 of a second (-1 EV) lets less light into the camera and produces a darker picture. At 1/250 (-4 EV) the image is way too dark.
I’m using shutter speed in this example, but we could just as easily have done the same thing by moving the aperture or ISO up and down.
Further, the settings all work together. So if you increase your shutter speed by one stop (subtracting an EV) but at the same time open your aperture by one stop (adding an EV), you end up back at the same exposure level you started with.
So why not just have one “EV control” setting? Why three? Because each of the thee points on the triangle has a different effect on your photos.
Shutter speed doesn’t have much effect on image quality unless the camera shakes – even a tiny bit – or something in the image moves. Naturally that’s a huge “unless” when using a hand-held camera in a world full of motion.
Aperture is a great way to let more light in, thus sparing yourself from wrestling with a super-slow shutter speed. However, the wider the aperture gets, the harder it can become to focus on more than one subect at once.
ISO is more difficult to adjust on many cameras. Though more light sensitivity can be a big help in low light situations, increasing the ISO can also decrease your image quality.