:: Depth of field
|Camera:||Canon EOS 5D Mark III|
What’s the point?
People ask all the time about where to place the focus point in a frame. If you’re a tourist shooter just grabbing snapshots, everything in your frame is likely at infinity and you can just put your camera in autofocus mode and click away. But if you’re an artistic photographer trying to capture something unique on your SLR camera, there’s a pretty good chance you have important objects at different distances throughout your frame. In that case you need to get out of autofocus mode and start taking control of your camera.
When focusing a composition, you first need to determine whether you even want sharpness throughout the frame. While some of my favorite images use selective focus to emphasize one element or area over another, right now I’m going to assume you’re looking to maximize your depth of field (DOF) to make everything in the frame sharp; I’ll save selective focus for another time.
I’m afraid that there’s some bad, albeit well-intended, advice out there that yields just enough success to lull people into thinking they’ve got focus nailed, a misperception that often doesn’t reveal itself until they ruin an important shot (or more). I’m referring to the myth that you should focus 1/3 of the way into the scene, or 1/3 of the way into the frame (two different things, each with its own set of problems). These techniques are useful rules of thumb if you clearly understand DOF and the art of focusing enough to adjust your focus point when appropriate, but once you achieve that level of understanding, you may as well do it right from the start.
Here are a few basic principles you should understand before you start making focus decisions (these may be review for many of you, so please bear with me):
- Aperture is the opening that allows light to reach your sensor–the bigger the opening, the more light gets in (duh), but also the narrower your DOF.
- Aperture is measured in f-stops. F-stop is a ratio (photographers conveniently remove the numerator for ease of reference and to sound smarter than we really are), so the higher the number, the smaller the aperture and the greater the DOF. That’s why f11 is actually a bigger aperture (with less DOF) than f16.
- Regardless of the current aperture setting, your SLR camera maximizes the light in its viewfinder by always showing you the scene at the lens’s widest aperture. All this extra light makes it easier to compose and focus, but unless you’re exposure is set for the widest aperture (which it shouldn’t be unless you have a very specific reason to do so), your image will have more DOF than you see in the viewfinder. In other words, you can’t see how much of your scene is in focus when you compose. Fortunately, most cameras have a DOF preview button that stops the lens down to its actual aperture–this allows you to see the actual DOF, but also darkens the viewfinder considerably, making it very difficult to see the scene you’re composing.
- The zone of focus sharpness extends a greater distance beyond the focus point than it does in front of the focus point. If you focus on that rock 10 feet in front of you, rocks five feet in front of you may be out of focus, but trees 100 feet away could be sharp. I’ll explain more about this later.
- The wider your focal length, the greater your DOF. In other words, you have more DOF at 20mm than you do zoomed to 100mm.
- Conversely, the closer your focus point, the narrower your DOF. If you focus on a butterfly on a poppy one foot in front of your lens, it’s possible that parts of the poppy are out of focus; if you focus on a tree 100 yards from your lens, it’s likely that the mountain behind the tree is sharp too. (The actual zone of sharpness will depend on your settings.)
- Foreground softness, no matter how slight, is almost always a greater distraction than slight background softness. So if it’s impossible to get all of your frame sharp, it’s usually best to ensure that the foreground is sharp.
The foundation of a sound approach to focus is hyperfocal focusing, a scientific method that yields the best (most precise and predictable) results. Hyperfocal focusing returns the range of “acceptable” sharpness in a scene for whatever combination of sensor size, focal length, aperture, and subject distance you use. Put as simply as possible, hyperfocal focusing establishes the depth of acceptable sharpness that extends from the midpoint between your lens and the hyperfocal point (the point where you focus), all the way out to infinity. Huh? In other words, if the hyperfocal point is twelve feet away, everything from six feet (half of twelve) to infinity will be acceptably sharp. We say “acceptably sharp” because there’s only one point of perfect sharpness (the hyperfocal point), but softness within the “acceptably sharp” range (the DOF) will not be significant (in theory).
The problem with hyperfocal focusing is that it requires in-the-field access to data that’s not necessarily conducive to the creative process–referring to charts or a smartphone app is awkward at best, and likely very distracting. I’m a big advocate of keeping photography as simple as possible, so while I’m a hyperfocal focus advocate in spirit, I don’t usually explicitly use hyperfocal data in the field. Instead I apply hyperfocal principles in the field. But to keep your eyes unglazed, I won’t dwell further on the technical minutia of hyperfocal focusing here. When you feel like you’re ready for more, Google “hyperfocal focus” to find tons of information and an array of charts, website tools, and smartphone apps.
Though I don’t use the data in the field, I find it helps a lot to refer to hyperfocal tables when I’m sitting around with nothing to do. So if I find myself standing in line at the DMV, or sitting in a theater waiting for a movie (I’m a great date), I open my iPhone app and plug in random values just to get a sense of the DOF for a given aperture and focal length combination. I may not remember the exact numbers later, but enough of the information sinks in that I accumulate a general sense of the hyperfocal DOF/camera-setting relationships.
Finally, something to do
Once my composition is worked out, I determine the closest object I want sharp. That can be pretty much anything with visual interest (shape, color, texture), regardless of whether it’s a primary subject. If it’s close enough to hit with my hat, I need a very small aperture (higher f-stop value) and opt for f16-f22 (and sometimes smaller if the lens allows it); if I could hit it with a baseball, I choose an f-stop between f11 and f16; if it would take a gun to reach it (infinity: imagine a distant peak), I choose an f-stop between f8 and f11. Of course these distances are very subjective and will vary with your focal length (not to mention your pitching arm), but they’re a good starting place.
Why not just automatically set my aperture at f22 and be done with it? I thought you’d never ask. Without delving too far into the physics of light and optics, let’s just say that there’s a little light-bending problem called “diffraction” that robs your images of sharpness as your aperture shrinks–the smaller the aperture, the greater the diffraction. So why not choose f4 when everything’s at infinity? Because lenses tend to lose sharpness at their aperture extremes, and are generally sharper in their mid-range f-stops. So while diffraction and lens softness don’t sway me from choosing the aperture that gives the DOF I want, I never choose an aperture bigger or smaller than I need.
Now that we’ve let the composition determine our f-stop, it’s (finally) time to actually choose the focus point. Believe it or not, now that you’ve built a foundation of understanding, focus becomes pretty simple. Whenever possible, I try to have elements throughout my frame, often starting near my feet and often extending far into the distance. When that’s the case I stop down to f16 (and sometimes smaller), compose on my tripod, and focus on an object slightly behind my closest subject (the farther my closest subject, or the wider my aperture, the farther behind it I can focus). If I’m not sure, or if I don’t think I can get the entire scene sharp, I err on the side of closer focus to ensure that the foreground is sharp. And often before shooting I check my DOF with the DOF preview button, allowing time for my eye to adjust to the limited light.
One of the great things about digital is the instant validation of the LCD–when I’m not sure, or when getting it perfect is absolutely essential, after capture I pop my image up on the LCD, magnify it to maximum, check the point or points that must be sharp, and adjust if necessary. Using this immediate feedback to make instant corrections really speeds the learning process.
Sometimes less is more
The depth of field you choose is your creative choice, and no law says you must maximize it. Use your camera’s limited depth of field to minimize or eliminate distractions, create a blur of background color, or simply to guide your viewer’s eye. Focusing on a near subject while letting the background go soft clearly communicates the primary subject while retaining enough background detail to establish context. And an extremely narrow depth of field can turn distant flowers or sky into a colorful canvas for your subject.
There’s no substitute for experience
No two photographers do everything exactly alike. Determining the DOF a composition requires, the f-stop and focal length that achieves the desired DOF, and where to place the point of maximum focus, are all part of the creative process that should never be left up to the camera. The sooner you grasp the underlying principles of DOF and focus, the sooner you’ll feel comfortable taking control and conveying your own unique vision.