In combination with Film Speed, aperture and shutter speed are the two controls used to determine the proper exposure for any given scene.

As with all things in Photography, there is in inverse relationship between aperture and shutter speed; in order to maintian balance (proper exposure), as one of these controls rises, the other needs to lower.

Aperture

The aperture controls the amount of light that enters a camera. Apertures exist in the lenses of cameras, and are usually controlled by a dial on the barrel of the lens, or via a dial on the body of electrically circuted and digital cameras. Regardless of where the aperture is controlled, they are always measured with the same numbers, called f stops

f/ 2.0
f/ 2.8
f/ 4
f/ 5.6
f/ 8
f/ 11
f/16
f/ 22

The smaller the aperture number (f stop), the bigger the opening. In other words, the smaller the f-stop, the more light shines on the film.

Depth of Field

A secondary property of aperture settings is the Depth of Field created by different f-stops. The Depth of Field is not the focus, but more sublte, the distance in front of and behind the plane of focus.

Read: Aperture Explained

Shutter Speed

In addition to the amount of light controlled by the aperture, the duration of light determined by the shutter speed is the other method by which to control an exposure.

In short, the faster the shutter speed, the less light shines on the film. The less light that shines on the film, the darker the exposure. Shutter speeds are measured in fractions of seconds as such:

1/4000
1/2000
1/1000
1/500
1/250
1/100
1/60
1/30
1/15
1/8
1/4
1/2
1”
2”
B

Stop Action vs. Motion Blur

A secondary property of shutter speeds are their ability to “freeze” action within the exposure with a fast shutter speed, and inversly, “blur” motion with a slow shutter speed.

Read: Shutter Speed Tediously Explained

Camera Shake

A note on using slow shutter speeds: Slower shutter speeds will blur any fast moving objects in your image. An (almost always) undesirable side-effect of using slow shutter speeds is that any motion you cause in the camera will yield an entirely blurry image due to camera shake. When you hand hold a camera, any shutter speed slower 1/60 will begin to show any movement of the camera you may cause during the exposure. Solution: Use a tripod or other means to rest your camera in a stationary position

Light Meter

Within your viewfinder, either below or beside the framed image will be your camera’s light meter. Light meters are designed differently among SLR’s, but all are made to provide feedback by evaluating the scene you are composing and giving you feedback, in terms of either aperture and/or shutter speed to help you properly compose your image.

Read: Kodak: Accurate Exposure with Your Meter

Exposure Settings Examined

1/6400 sec. @ f/2.5

1/30 sec. @ f/22

This slow shutter speed not only blurs Prada, it also creates camera shake. Note how even at a very small aperture the image is still overexposed due to the lengthy exposure time.