A lens’ aperture works exactly like the pupil of an eye. The wider it is, the more light it lets in. The more light it lets in, the shorter the shutter speed needs to stay open to properly expose a picture. The figure given below shows approximate relative sizes of the most commonly used apertures. Though the diagram shows the apertures as perfect circles, most lenses have apertures with 6-9 “blades,” which make almost-circular shapes with the same number of sides.
Aperture is measured in f/stops, such as f/2.8, f/8, and f/16. It seems counter-intuitive, but the smaller the f/stop number, the wider the aperture. Therefore, f/2.8 is a much wider aperture than f/8 and f/16.
Besides controlling the amount of light that enters the lens, aperture is the easiest way to control depth-of-field. With a short depth-of-field, out of focus subjects are very blurry. With a long depth-of-field, out of focus subjects can appear to be in focus.
Small f/stop numbers are especially useful for portrait photography. While greater f/stop numbers are especially useful for landscape photography because in landscape photographer always need to focus on every single detail.
The only way to really understand aperture and depth-of-field is practice. To remember which way to adjust the aperture, remember this: low f/stop number, low background sharpness; high f/stop number, high background sharpness. Figuratively speaking, depth-of-field is not black-and-white, but shades of grey. While your camera always focuses on a single focal plane, subjects directly in front of and behind the focal plane will be almost in focus. Move a bit closer to or farther away from the camera, and subjects will be slightly less in focus. The farther something is from the focal plane, the more out of focus it will be.
Controlling Aperture Using Automatic Modes
To control the aperture using the automatic modes on your camera (which are also available on point-and-shoot cameras), select either Portrait or Landscape mode:
- Portrait mode chooses the smallest f/stop number possible, providing the shallowest depth-of-field, making the background blurry.
- Landscape mode chooses the largest f/stop number possible, providing the deepest depth-of-field, making the background
If you want a nicely blurred background, try this:
- Choose portrait mode.
- Zoom all the way in.
- Get as close to the subject as your camera will focus.
- Choose a location with a distant background.
Many point-and-shoot cameras are designed with lenses that do not support the small f/stop numbers required to blur the background nicely. If background blur is important to you, consider using a DSLR with a lens that supports f/stop numbers of f/2.8 or smaller.
Controlling Aperture Using Creative Modes
To control the aperture using the creative modes on your SLR, select aperture priority (A or Av) mode. Then, move the main dial (the small dial next to the shutter button) to select the desired aperture. Note that as you choose smaller f/stop numbers to blur the background, your shutter speed increases, and as you choose higher f/stop numbers to sharpen the background, your shutter speed decreases. The apertures you can choose from are defined by your lens. The lowest f/stop number is so important that they put it right into the lens’ name.
Many DSLRs include a depth-of-field preview button that shuts the aperture down to the setting you’ve selected. If you press the button, you’ll notice that the viewfinder immediately gets dark—that’s the effect of using a higher f/stop number. If the viewfinder isn’t too dark to see clearly, you’ll also notice that the depth-of-field increased.