Three Pillars of Exposure (Part 3): Aperture

Imagine zooming in on the fine details of the pistil and stamens of a flower. Or trying to capture the veins of a butterfly’s wings. Or stepping back to shoot the beautiful vista of a mountain valley. All of these instances will have you considering aperture. Aperture is one of the three pillars of exposure that photographers learn to master in order to make their work outstanding.

Part 1 of this blog series discusses ISO. Part 2 covers shutter speed. And now in Part 3, the final blog in the Three Pillars of Exposure, we’ll cover aperture.

What is aperture?

Aperture refers to the opening in the lens.

To clarify, where shutter speed refers to what’s going on in the camera body, aperture refers to the opening of the diaphragm in the lens.

When you press the shutter release button, a tiny circular hole opens in the lens to allow light to pass onto the camera sensor. You can control the diameter size of that opening, allowing more or less light to pass through, by adjusting the aperture setting.

Aperture is measured in f-stops. It’s often listed as f/number. For example f/2.8, f/4, and f/22.

The crucial thing to remember, that may be confusing at first, is that the higher the number of the f-stop, i.e. f/22, the smaller the opening (and less light will pass through). Therefore, f/22 is a smaller aperture than f/4 because f/22 has a smaller opening in the lens, which will allow a much smaller amount of light onto the sensor.

Tip: The higher the number of the f-stop, the smaller the opening (and less light will pass through).

Moving the f-stop to the next number in your settings will halve or double the amount of light, depending whether you’re moving it up or down. This is similar to ISO settings and shutter speed settings, where moving to the next number may also halve or double the amount of light coming through. Therefore increasing one setting while decreasing the other setting will give you the same amount of exposure. Key point to remember. This is why these elements are referred to as the three pillars of exposure.

Why would you ever wish to increase or decrease aperture? To control the depth of field (DOF). In other words, to control how much of your photograph is in focus.

If you want very little in focus, or a very shallow DOF, keep the f-stops on the larger apertures, for example from f/1.4 to f/4. These kind of shots might include macro photography such as insects and flowers. The detail will be sharply in focus, but the rest of the photograph will be beautifully blurry (in photography lingo, this nice blurry background is called “bokeh”). A shallow DOF keeps the focus on the subject, and minimizes the distractions of the background elements. If you’d like most of the photo in focus, keep the f-stops on the smallest apertures, toward the f/22 setting. If you’d like something in between, you might use f/5.6 to f/8.

Here’s the handy rule of thumb I use:

Shallow Depth of Field
Medium Depth of Field
Large Depth of Field
such as macro shots of insects, flowers such as fields of flowers, waves on a beach

such as mountains, valleys, oceans

f/1.4 to f/4  f/5.6 to f/8  f/11 to f/22

Have a look at your lens barrel. On most DSLR lenses, the maximum widest aperture possible for that lens will be listed on the lens barrel. For example, a zoom lens of 18-55 mm might list the aperture as a range of 3.5-5.6. This means 3.5 is the maximum widest aperture at 18 mm, and 5.6 is the maximum widest aperture at 55 mm. Some high end lenses can maintain the maximum widest aperture for all ranges of the zoom. For example, a 70-200mm lens might have 2.8 listed on the lens barrel. A lens with a large aperture, such as 2.8, is often referred to as a “fast” lens.

Another interesting thing to remember is that all DSLR cameras have a “sweet spot”, where the aperture setting works at the camera’s absolute best, making the focus you’re aiming at tack sharp. For most cameras, it’s usually around f/5.6 or f/8. If you’re trying to get a certain magnificent shot, you might keep that in mind. It’s always great to know what your camera is capable of doing.

Have you discovered or observed your camera’s sweet spot of focus, when it comes to aperture?

Image credit: