Depth of field
What’s the point?
It seems like one of photography’s great mysteries is achieving proper focus—the settings to use, where to place the focus point, even the definition of sharpness are all sources of confusion. If you’re a tourist just grabbing snapshots, everything in your frame is likely at infinity and you can just put your camera in full auto mode and click away. But if you’re an artistic photographer trying to capture something unique with your DSLR or mirrorless camera, you should be doing your best to have important objects at different distances throughout your frame. In that case you need to get out of the auto modes and start taking control of your exposure settings and focus point.
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 and blur the rest of the scene, most (but not all) of what I’ll say here is about maximizing depth of field (DOF). I cover creative selective focus in much greater detail in another Photo Tip article: Creative Selective Focus.
Beware the “expert”
I’m afraid that there’s some bad, albeit well-intended, advice out there that yields just enough success to deceive people into thinking they’ve got focus nailed, a misperception that often doesn’t manifest until an important shot is lost. 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 very different things, each with its own set of problems).
For beginners, or photographers whose entire scene is at infinity, the 1/3 technique may be a useful rule of thumb. But taking the 1/3 approach to focus requires that you understand DOF and the art of focusing well enough to adjust your focus point when appropriate, and once you achieve that level of understanding, you may as well do it the right way from the start.
Back to the basics
Understanding a few basic focus truths will help you make focus decisions:
- A lens’s aperture is the opening that allows light to reach your sensor—the bigger this opening, the more light gets in, but also the smaller your DOF.
- Aperture is measured in f-stops, which is the lens’s focal length divided by the aperture’s diameter; the higher the f-number, the smaller the aperture and the greater the DOF. So f/8 is actually a bigger aperture (with less DOF) than f/11. Some find it helpful to think of f/stops this way: The higher the f-number, the greater the depth of field. Though they’re not exactly the same thing, photographers usually use f-stop and aperture interchangeably.
- Regardless of the current f-stop setting, a 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 your exposure is set for the widest aperture (which it shouldn’t be unless you have a very specific reason to limit your depth of field), the image you capture will have more DOF than you see in the viewfinder. The consequence is that you usually can’t see how much of your scene is in focus when you compose. Most cameras have a DOF preview button that temporarily closes the lens down to the f-stop you have set—this shows the scene at its actual DOF, but can also darken the viewfinder considerably (depending on how small your aperture is), making it far more difficult to see the scene.
- 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 ten feet in front of you, rocks three feet in front of you may be out of focus, but a tree fifty feet away could be sharp. I’ll explain more about this later.
- While shorter focal lengths may appear to provide more depth of field, DOF doesn’t actually change with focal length. What does change is the size of everything in the image, so as your focal length increases, so does your apparent DOF. So you really aren’t gaining more DOF with a shorter focal length, but softness will become more apparent as your focal length increases because your subjects are magnified.
- The closer your focus point, the narrower your DOF (range of front-to-back sharpness). If you focus on a butterfly sunning on a poppy one foot in front of your lens, your DOF is so narrow that it’s likely parts of the poppy will be 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. (But the actual zone of sharpness will vary with your sensor size, f/stop, and focal length.)
- 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.
Depth of field discussions are complicated by the fact that “sharp” is a moving target that varies with print size and viewing distance. But it’s safe to say that, all things equal, the larger your ultimate output and closer intended viewing distance, the more detail your original capture should contain.
To capture detail, a lens focuses light on your sensor’s photosites. Remember using a magnifying glass to focus sunlight and ignite a leaf when you were a kid? The smaller (more concentrated) the point of sunlight, the sooner the smoke appeared. In a camera, the finer (smaller) a lens focuses light on each photosite, the more detail the image will contain.
In photography we call that small circle of light your lens makes for each photosite its “circle of confusion.” The larger the CoC, the less concentrated the light and the more blurred the image will appear. Of course if that large CoC is still too small to be seen as soft, it really doesn’t matter. In other words, areas of an image with a large CoC (relatively soft) can still appear sharp if viewed from far enough away. That’s why sharpness can never be an absolute term, and we talk instead about “acceptable sharpness” that’s based on print size and viewing distance.
But how much detail do you need? For any focus point, there’s only one plane of maximum sharpness, regardless of the focal length and f-stop—everything in front of and behind the plane of your focus point will be some degree of less than maximum sharpness, and the farther in either direction a point is from the focus plane, the larger the CoC and the softer it will appear. But depending on the viewing distance, this less than maximum sharpness can still be acceptable. Every point in the image with a CoC that’s too small for your eye to discern will be acceptably sharp.
Put in more practical terms, the threshold for acceptable sharpness is pretty low for an image that just ends up on an 8×10 calendar on the kitchen wall, but if you want that image large on the wall above the sofa, achieving acceptable sharpness requires much more detail. And as your print size increases, the CoC that delivers acceptable sharpness shrinks correspondingly.
Many factors determine the a camera’s ability to record detail. Sensor resolution of course—the more resolution your sensor has, the more important it becomes that to have a lens that can take advantage of that extra resolution. And the more detail you want to capture with that high resolution sensor and tack-sharp lens, the more important your depth of field and focus point decisions become.
The foundation of a sound approach to achieving proper focus for your viewing distance and image size is hyperfocal focusing, a science-based method that uses viewing distance, f-stop, focal length, and focus point to ensure acceptable sharpness for a given CoC.
The focus point that provides the maximum depth of field for a given combination of f/stop and focal length is the hyperfocal point. When focused at the hyperfocal distance (on the hyperfocal point), your scene will be acceptably sharp from halfway between your lens and focus point all the way to infinity. For example, if the hyperfocal point is twelve feet away, focusing there will give you acceptable sharpness from six feet (half of twelve) to infinity—focusing closer will soften (increase the CoC for) the distant scene; focusing farther will extend the area of foreground softness.
The most overlooked aspect of hyperfocal focusing is determining the proper CoC. Since people’s eyes start to glaze over when CoC comes up, they tend to use the default given to them by their table or smartphone app. At the very most they plug in their sensor size (a good start) and consider themselves done.
What you may not know is that the hyperfocal tables and focus distance markers on prime lenses (remember those?) that have been plugged into most hyperfocal smartphone apps assume that you’re creating an 8×10 print that will be viewed from a foot away. Worse still, they have applied a CoC that is three times larger than the eye’s ability too resolve.
That doesn’t invalidate hyperfocal focusing, but if you use published hyperfocal data (or a hyperfocal app), your images’ DOF might not be as great as you think it is (for your need). If you can’t specify a smaller CoC in your app, I suggest that you stop-down a stop or so more than the app/table indicates. On the other hand, stopping down to increase sharpness is an effort of diminishing returns, because diffraction increases as the aperture shrinks and eventually will soften the entire image—I try not to go more than a stop smaller than my data suggests.
Another downside to 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, and likely 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 use hyperfocal data in the field. Instead I apply hyperfocal principles in the field.
Though I don’t often use the specific hyperfocal 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 hyperfocal app and plug in random values just to get a sense of the DOF for a given f-stop 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
Unless I think I have very little DOF margin for error in a composition, I rarely open my hyperfocal app in the field. Instead, 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 and I want sharpness to infinity, I need lots of DOF and opt for f/16 (if I fear I might need to go smaller than f/16, I usually pull out my app to verify); if I could hit it with a baseball, I choose an f-stop around f/11; if it would take a gun to reach it (infinity: imagine a distant peak), I choose an f-stop between f/8 and f/11. Of course these distances are very subjective and will vary with your focal length and composition (not to mention the strength of your pitching arm), but you get the idea.
Why not just automatically set my aperture to f/22 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 no so little light-bending problem called “diffraction” that robs your images of sharpness as your aperture shrinks—the smaller the aperture, the greater the diffraction. Then why not choose f/2.8 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 f-stop that gives the DOF I want, I try to 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, with this foundation of understanding we just established, focus becomes pretty simple. Whenever possible, I try to have elements throughout my frame, often starting near my feet and extending far into the distance. When that’s the case I stop down focus on an object slightly behind my closest subject (the more distant my closest subject, the farther behind it I can focus).
When 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. Sometimes before shooting I check my DOF with the DOF preview button, allowing time for my eye to adjust to the limited light. And when maximum DOF is essential and I know my margin for error is small, I don’t hesitate to refer to the DOF app on my iPhone.
A great thing about digital capture 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.
We cover DOF in much more detail in my photo workshops
Playing with depth
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