Missing Pages: Depth of Field
The Missing Pages column is a collection of all of the information that should have been included in your camera’s Owner’s Manual—but somehow got left out. This is a hybrid assortment of short articles that delivers the know-how you need to derive the maximum enjoyment—and creative expression—from your equipment.
It’s sort of a juiced-up User Guide for creative people who are not necessarily technical. Each part will teach you how to use one of the camera features or functions that you previously ignored or left set on Auto. And each will include a Creative Project so that you can try some scripted experimentation.
We will explain complex technical subject matter a way that everyone can understand. And if you happen to be a technical expert yourself, we’re including “Nerds Only” sidebars just for you. That way you can dig in deep—or just straddle the edges—of the technological stuff. It’s your decision.
Installment V: Depth of Field
Definition: The area in front of and behind the point of focus that also appears to be sharp.
You’ve seen the classic portrait that depicts a smiling face in sharp focus floating over a blurred and obscured background, as in the photo below. The subject appears to pop right off the page. And you’ve seen hundreds of shots of landscapes where the animals in the foreground are as sharp and clear as the mountains behind them. How can you control exactly what’s in focus and what’s not—particularly with an autofocus camera?
The phenomenon at work here is called Depth of Field (DOF). Because it’s predictable, reproducible and measurable we can use it to our best advantage. And since it’s been studied by scientists and optical engineers for centuries, we know a lot about it.
DOF increases as the diameter of the aperture decreases; in other words, a smaller f/stop (like f/16) generates greater DOF than a larger f/stop (like f/2.8). Sometimes people confuse this with sharpness but the two are quite different. In fact, using a very small aperture can rob you of sharpness. You can learn more about that by reading the Nerd’s Only section below.
One way to control DOF is by using the right aperture, the aperture that delivers the amount of DOF that you’re after. Shooting in Program or Auto Exposure mode surrenders control over the f/stop, so your camera must be set on Aperture Priority or Manual if you wish to control DOF. Some lenses have a Depth of Field scale printed on the barrel. It looks like a symmetrical set of hash marks that delineate the range of focus when aligned with the aperture that’s being used and the distance from the subject.
This is a very reliable way of determining DOF. Unfortunately, it is accurate without being precise. The increments are simply too inexact. As you can see in the image above, this particular scale skips from 1 meter to 1.5 meters to 3 meters to infinity. And you won’t find a scale on a zoom lens since the focal length is literally a moving target.
Optical engineers have created DOF charts for many different lenses. Many high quality lenses include these charts in the owner’s manual. DOF can be calculated mathematically using the circle of confusion for a given lens as one of the variables. The term “circle of confusion” describes a spot of light that is slightly out of focus when a lens is focused on a given point. You do not need to know anything about circle of confusion to control DOF.
Some digital SLR cameras have a DOF preview button. Even if your camera does have one, don’t trust it 100%. The DOF that your eye perceives when looking through the viewfinder differs from what the camera sees, even when the lens is stopped-down to the shooting aperture. It’s a close approximation, but not identical. Use a DOF chart or the scale described above.
Wide angle lenses produce more DOF than telephoto lenses (assuming that film format or sensor size is the same). Some will argue that if you crop a WA image to match the field-of-view of a telephoto lens the DOF will commensurately diminish. So what? The point remains: put a wideangle lens on your camera and you’ll enjoy more Depth of Field.
Depth of Field always extends 1/3 in front of and 2/3s behind the point of focus. No matter whether the DOF is deep or shallow, it always follows this formula. This fact becomes more valuable when you do macro photography.
Depth of Field decreases as the distance between the subject and film plane decreases. You have VERY little DOF to work with when doing macro photography and are focused just a couple centimeters away, but you have extreme DOF when focused at a point near infinity.
All lenses have a Hyperfocal Distance for a given f/stop. If, for example, the Hyperfocal Distance happens to be 16 feet for a particular lens/aperture combination, everything from one-half that distance (8 feet) to infinity appears to be in focus. If your lens has a DOF scale as described above, line up the infinity symbol with the f/stop you are using and you have just set your lens to its Hyperfocal Distance for that f/stop.
Depth of Field sounds like a good thing and usually it is—but not always. If you want to produce dramatic portraits you’ll want to limit DOF, as in the image at the top of this story. By limiting DOF you can make a lens appear sharper. Can you guess why?
Boke (sometimes spelled bokeh) is pronounced “bow” as in “beau” plus “keh” rhyming with the first “e” in “edible.” In Japanese is means “fuzzy” and in photography it’s used to describe the parts of a photograph that are not in focus. Boke can also be used to describe people with fuzzy behavior, but that’s another story. Anyway, some lenses are optimized to produce attractive boke. Your assignment is to explore boke and how it relates to DOF.
You’ll need a lens with a fairly fast aperture: f/3.5 or faster. Position a human subject at least two meters from a background that’s filled with bright colors and small detail (for example, the wall of snazzy-looking cereal boxes one sees at a grocery store).
Focus on your subject from a distance that allows their shape to occupy about 50% of the frame and shoot with your lens wide open (largest aperture) in Manual or Aperture Priority. Repeat these steps, but this time stop the lens down three full stops. Now compare how the out-of-focus backgrounds are rendered.
Try the same experiment outdoors using a background of flowers or trees, preferably with a bit of backlight filtering through (so that you can blur a few point sources and highlights).
For Nerds Only
Nearly every aspect of photography involves achieving a balance between competing forces. DOF competes with sharpness and diffraction is the culprit.
Diffraction occurs when light encounters an obstacle that it cannot penetrate. Light travels in waves that are parallel when unobstructed. When light waves pass through an aperture they begin to diverge and interfere with one another. The smaller the opening, the greater the interference. Light waves that are traveling divergent paths are, by definition, traveling different distances and becoming out-of-phase overall. So at some point, an aperture that is “too small” will cause the photo to be unsharp, even though it will have tremendous Depth of Field. To get the most from your lenses you should know the “sweet” aperture, the f/stop that delivers the best image with the fewest aberrations. And you should know at what point diffraction begins to rob your lens of sharpness.
Text ©2009 Jon Sienkiewicz.