Dec 14 2009

Out of Your Depth?
Exposing Aperture, Shutter Speed, and Depth of Field

Reasonably priced digital cameras have encouraged a lot of people to rethink their attitudes toward picture-taking. Some are returning to photography after a long absence. Others are discovering its pleasures for the first time. And what is it about digital cameras that makes such a difference? Well, they have many advantages, including:

  • Freedom from the costly tyranny of film and photo-processing
  • Ease of use: you can point and shoot with confidence
  • Versatility (some digicams shoot decent video, with sound)
  • Control over the final product, both before and after the shot

Most digital cameras let you choose an Automatic mode that makes all the decisions for you. That’s fine if you’re happy just to capture snapshots, but if you want more than that, you’ll need to know how your camera functions and how to get the most from it, so let’s…

Explore the Interplay Between Aperture and Shutter Speed A photograph is made by exposing sensors to light for a predetermined period of time. There are two variables at work here: the size of the tiny window that admits light to the camera and the length of time the shutter stays open. Getting the balance between these two elements just right makes all the difference between a good photo and one that’s either too light or too dark. The image’s tonality—whether it’s light or dark—is determined by the camera’s opening (aperture or ƒ-number) and its shutter speed. Shutter speed takes priority if what you want to do is freeze or blur action. On the other hand, if you’d like to control how much of a scene is “in focus”—a little, a lot, or all—then aperture is the number one consideration. This is because aperture determines the picture’s depth of field (aka DoF). Confused? No surprise there. It is confusing. And that’s why Automatic mode is so popular with casual photographers. You can allow the camera to choose the ƒ-number and shutter speed for you. But to move beyond the snapshot stage you’ll need to take control yourself. This brief run-down should help:

Aperture In photography, aperture is indicated by ƒ-number. The higher the ƒ-number, the smaller the aperture. Since aperture determines the size of your camera’s window on the world, and since big windows let in more light than little windows, an aperture of ƒ/5.6 lets in more light than an aperture of, say, ƒ/22. So far, so good. But as I just noted, there’s more to aperture than light. Aperture also determines…

Depth of Field Here’s where things get even more confusing. The higher the ƒ-number, the greater the depth of field, the name given to the distance between the nearest and most distant points that a lens can bring into acceptably sharp focus simultaneously. Of course, the inverse is also true: the smaller the ƒ-number, the shallower the depth of field.

Aperture on 18-55 mm Digilens

The pair of pictures above shows the extremes of aperture when my Pentax-DA 18-55mm f/3.5-5.8 AL II lens is set at the 18mm focal length. What you’re seeing on the left is the view down through the lens with the aperture at ƒ/3.5; the right-hand shot shows the lens at ƒ/22. Small ƒ-number, large opening. Large ƒ-number, small opening. It’s as simple as that, but if you’d want to explore this subject in depth, see the Wikipedia entry.

Now let’s look at…

Shutter Speed A shutter inside your camera opens and closes when you trip the release button. During the time that it’s open, the shutter allows light to reach the camera sensors, and this interval is called the shutter speed. A fast shutter speed (also called a “short” shutter speed) limits exposure time and freezes action; a slow (or long) shutter speed does the opposite, turning action scenes into a blur of movement— “motion blur,” in trade jargon. To avoid problems with the shakes when shooting without a tripod, it’s essential to keep shutter speeds well under a second. In fact, with a “normal” lens, it’s best to avoid exposures longer than 1/30th of a second.

Shutter speed and aperture need to be balanced against each other to achieve the tonality you desire. You don’t want to overwhelm your camera’s sensor with too much light. So if you need a slow shutter speed—to blur fast-moving water, say—you’ll need to close down the aperture to reduce the size of the camera’s window on the world. Here’s a pair of pictures showing how aperture (ƒ-number) and shutter speed relate to one another and to depth of field:


Got it? No? Then here’s a summary:

  • Small ƒ-number = Faster shutter speed = Shallow DoF
  • Large ƒ-number = Slower shutter speed = Deep DoF

Now let’s consider a range of alternatives, beginning with…

Prioritizing Depth of Field Here’s a photo showing a cluster of chokecherries in sharp focus in the foreground. With a narrow DoF, however, the farther cluster is visible only as a blurry outline:

Limited DoF

Limited DoF is often artistically appealing, and the blurring is called bokeh. In the picture above, it hints at abundance without diverting the viewer’s eye from the primary subject. The photo was shot at close range with a relatively wide angle of 35mm, using my Canon point-and-shoot PowerShot at ƒ/5.6 and 1/60 second. Here’s another photo shot at the same aperture and speed, but with my Pentax K200D DSLR and a focal length of 200mm:

Mini Forest

I wanted to isolate the tiny club moss and gilled mushroom atop a mossy stump by blurring foreground and background, so I opted for the smallest ƒ-number (and the narrowest DoF). Only then did I select the shutter speed. To avoid the shakes and maintain a sharp focus on my main subject, I braced both my body and the camera against a stout tree.

Now let’s look at the other end of the DoF spectrum:

Ledge Drop

This photo was shot with my DSLR and 18-55mm lens, zoomed down to 18mm. I set the aperture at ƒ/22 and the exposure at 1/2 second (I used a tripod). My intent was to render the moving water as a silky blur, but I also wanted to keep the whole scene in focus. So I set the aperture first and then selected the shutter speed. The high ƒ-number insured that the latter would be slow enough to blur the water. I was pleased with the result. But there are times when my main concern is to freeze action, and that’s…

When Shutter Speed Rules If your primary goal is to freeze fast action, shutter speed is more important than depth of field. In the next picture, I wanted to compress the stepped falls to give a sense of drama, and I also wanted to freeze the flow of water. I shot the picture with a telephoto zoom lens set to 125mm (equivalent to a 187mm telephoto on a 35mm film camera) and chose a shutter speed of 1/500 second. Only then did I select the aperture—ƒ/8.

Stepped Falls

In the final shot, I wanted to capture the antics of a chipmunk foraging on a dark, overcast day. Because I had my DSLR’s ISO set at a low 100 (to avoid “noise” in the shadows), the fastest shutter speed I could use was 1/60, and the aperture needed to keep the scene bright enough was ƒ/5.6. That setting was fine for capturing the chipmunk in a sitting position, but when she moved, 1/60 second didn’t come close to freezing the action. Still, the resulting blur suits this speedy little animal and emphasizes her frenetic nature.

Busy Chipmunk

By understanding the interplay between aperture, shutter speed, and depth of field, you can take control of your photos. Aperture controls depth of field, and this is the primary concern when you wish to draw the eye to a particular part of a scene. If a narrow emphasis is what’s wanted, a narrow depth of field will do it for you, and you’ll need a small ƒ-number. If, on the other hand, you want everything in focus from the near foreground to the far distance, a higher ƒ-number will give you what you want. Finally, if your goal is is to stop the action (or intentionally blur it), then shutter speed will be of more importance than aperture, and you should choose accordingly.

Experiment with your camera’s settings. Pick a scene or subject and try limiting and widening depth of field by turns, then repeat the process, giving priority to shutter speed. Look at the results and judge which ones are best for which purpose. Keep notes, or consult the images’ EXIF data. Carry the exercise on to other subjects and scenes, and keep comparing the results. Soon you’ll have learned how your camera’s settings can be made to work for you, and before you know it you’ll no longer be out of your depth.

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