Depth of Field

What’s the point?

It seems like one of landscape photography’s great challenges is juggling the variable required to achieve proper focus: the camera settings (focal length, f-stop, and sometimes even shutter speed and ISO), where to place the focus point, even the definition of sharpness. 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 to your heart’s content. But if you’re a photographic artist trying to capture something unique with your mirrorless or DSLR camera, while doing your best to keep sharp important visual elements at different distances through your frame, you need to stop letting your camera decide your focus point and exposure settings.

Of course the first creative focus decision is whether you even want the entire frame sharp at all. 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 using hyperfocal techniques to maximize depth of field (DOF). I cover creative selective focus in much greater detail in another Photo Tips 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).

Focus control is especially important in those scenes where missing the focus point by just a few feet, or even inches, can make or break the image. For beginning photographers, applying the 1/3 technique is better than nothing. But taking the 1/3 approach to focus requires that you understand DOF and the art of focusing well enough to know when 1/3 doesn’t work, and how to adjust your focus point and settings accordingly. 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 ratio of a lens’s focal length divided by its 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. This understanding becomes second nature, but if you’re just learning it’s 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 its current f-stop setting, a DSLR 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 or maximize the light), when you snap the picture the aperture will close and the image you capture will have more DOF than you see in the viewfinder. The consequence is that, with a DSLR, you usually can’t see how much of your scene is in focus when you compose (advantage mirrorless). 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 also darkens the viewfinder considerably (depending on how small your aperture is), making it far more difficult to see the scene.
• Regardless of the focal length and f-stop, there’s only one (infinitely thin) plane of perfect sharpness for any focus point—everything in front of and behind the plane containing your focus point (and parallel to the sensor) will be some degree of less than maximum sharpness. As long as the zone of less than perfect sharpness isn’t perceptible, it’s considered “acceptably sharp.” When that zone becomes visible, that portion of the image is officially “soft.” Acceptable sharpness varies with the display size and viewing distance.
• The zone of acceptable sharpness extends a greater distance behind 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 5 feet in front of you may be out of focus, but a tree 20 feet away could be sharp. I’ll explain more about this later.
• While shorter focal lengths may appear to provide more depth of field, believe it or not, 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, your apparent DOF decreases. So you really aren’t gaining more absolute DOF with a shorter focal length, the softness just won’t be as visible—in other words, a wider focal length increases the range of acceptable sharpness. When photographers talk about DOF, they’re virtually always talking about apparent DOF (acceptable sharpness)—the way the image looks. (That’s the DOF definition I use here too.)
• The closer your focus point, the narrower your DOF (range of front-to-back sharpness). If you focus your 50mm lens on a poppy 6 inches from your lens, your DOF is so narrow that it’s possible parts of the poppy will be out of focus; if you focus the same lens on a tree 100 feet away, the mountains behind the tree could be sharp too.

Defining sharpness

Depth of field discussions are complicated by the fact that “sharp” is a moving target that varies with display size and viewing distance. But it’s safe to say that all things equal, the larger your ultimate output and closer the intended viewing distance, the more detail your original capture should contain. This fact is complicated further by the ridiculous resolution of today’s cameras, which makes it increasingly easier to create huge images with detail that can be viewed at close distances.

To capture detail, a lens focuses light on the 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 at that location. So when we focus, we’re trying to concentrate the light striking each photosite as much as possible.

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 the CoC is too small to be seen as soft, either because the print is too small or the viewer is too far away, it really doesn’t matter and you have an acceptably sharp image. In other words, areas of an image with a large CoC (relatively soft) can still appear sharp if it’s small enough, or 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. It’s actually possible for the same image to be sharp for one use, but too soft for another.

So how much detail do you need? The threshold for acceptable sharpness is pretty low for an image that just ends up on an iPhone or an 8×10 calendar on the kitchen wall, but if you want that image to fill the wall above the sofa, acceptable sharpness requires much more detail. And as your print size increases (and/or viewing distance decreases), 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.

Hyperfocal focus

The foundation of a sound approach to maximizing sharpness for a given viewing distance and image size is hyperfocal focusing, an approach that uses viewing distance, f-stop, focal length, sensor size, and focus point to ensure acceptable sharpness.

The hyperfocal point is the focus point that provides the maximum depth of field for a given combination of sensor size, f/stop, and focal length. Another way to express it is that the hyperfocal point is the closest you can focus and still be acceptably sharp to infinity. When focused on the hyperfocal point, your scene will be acceptably sharp from the point halfway between your lens and that focus point, all the way to infinity. For example, if the hyperfocal point for your sensor/focal length/f-stop combination is 12 feet away, focusing there will give you acceptable sharpness from 6 feet (half of 12) to infinity—focusing closer will soften the distant scene; focusing farther will keep you sharp to infinity but extend the area of foreground softness away from your camera.

Because the hyperfocal-variable (sensor size, focal length, f-stop) combinations are too numerous to memorize, we usually refer to an external aid. That used to be awkward printed tables with long columns and rows displayed in microscopic print—the more precise the data, the smaller the print. Fortunately, those have been replaced by smartphone apps with more precise information in a much more accessible and readable form. We plug in all the variables and out pops the hyperfocal point distance and other useful information

It usually goes something like this:

1. Identify the composition
2. Determine the closest thing that must be sharp (right now I’m assuming you want sharpness to infinity)
3. Dig your smartphone from one of the 80 pockets it could be in (let’s all agree that there are too many pockets in photography)
4. Open the hyperfocal app and plug in the sensor size (usually previously set by you as the app’s default), f-stop, and a focus distance. Some apps require you to input a focus distance, but this is irrelevant to determining the hyperfocal point (but essential once you have the hyperfocal distance and are actually ready to focus).
5. Up pops the hyperfocal distance (and usually other info of varying value)

You’re not as sharp as you think

Since people’s eyes start to glaze over when circle of confusion comes up, they tend to use the default returned by the smartphone app. But just because the app tells you you’ve nailed focus, don’t assume that your work is done. An often overlooked aspect of hyperfocal focusing is the fact that your app makes assumptions that aren’t necessarily right, and in fact are probably wrong.

As I said earlier, the CoC your app uses to determine acceptable sharpness is a function of sensor size, display size, and viewing distance. But most app’s hyperfocal tables use now obsolete data that date back to the film days and assume that you’re creating an 8×10 print that will be viewed from 12 inches away—maybe valid 50 years ago, but not in this day of high resolution mega-prints. The result is a CoC 3 times larger than the eye’s ability to resolve.

That doesn’t invalidate hyperfocal focusing, but if you use published hyperfocal data from an app or table, your images’ DOF might not be (probably aren’t) as ideal as you think it is for your use. If you can’t specify a smaller CoC in your app, I suggest that you stop-down a stop or more beyond what the app/table indicates.

On the other hand, stopping down to increase sharpness is an act of diminishing returns, because diffraction increases as the aperture shrinks and eventually will soften the entire image. So in a perfect world, it’s also important not to stop down too far. I only (unless I forget to reset my f-stop from my previous composition, which definitely happens) use an f-stop smaller than (larger number) f/11 when the situation calls for it, and rarely choose anything smaller than f/16.

Keeping it simple

As helpful as a hyperfocal app can be, whipping out a smartphone for instant in-the-field access to data is not always conducive to the creative process. 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 whenever I think the margin of error gives me sufficient wiggle room.

On the other hand, while 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 Starbucks or the DMV, I open my 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

The key to hyperfocal focusing is understanding that you should never focus on the closest thing that must be sharp. Why not (I thought you’d never ask)? Let me explain…

Since you always have some degree of acceptable sharpness in front of your focus point, and an even greater range of sharpness behind it, focusing on the closest thing that needs to be sharp gives you irrelevant foreground sharpness in front of your focus point, at the expense of distant sharpness you very much might miss.

So once my composition is worked out, I determine the closest object that must be sharp—a rock, flower, shrub, or whatever. This doesn’t need to be a subject of significance, just something that might draw the eye. Then I pick a focus point that’s a little bit behind the closest thing that needs to be sharp. How much is a little bit? If I don’t feel like I have much DOF margin for error, I whip out my hyperfocal app to answer that question. But when my focal length is fairly wide and I feel like I have enough DOF wiggle room to keep my hyperfocal app in my pocket, I go through the following mental checklist to determine how far behind my closest object to focus:

• If my closest foreground object (that needs to be sharp) is close enough to hit by tossing my hat, I need a fair amount of DOF. If my focal length is relatively wide (35mm or wider), I might stop down to f/16, and focus a foot or two behind my foreground object.
• If I could hit my foreground object with a baseball and my focal length is less than 100mm, I’ll probably go with f/11 and focus 10-20 feet behind my closest foreground object.
• If it would take a gun to reach my closest object (imagine a distant peak), I choose an f-stop between f/8 and f/11 and focus anywhere in the distance.

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. If you find yourself in a small margin for error focus situation without a hyperfocal app (or you just don’t want to take the time to use one), the single most important thing to remember is to focus behind your closest subject.

And finally, foreground softness, no matter how slight, is almost always a greater problem than slight background softness. So, if it’s impossible to get all of your frame sharp (and you’re like me and don’t use focus stacking), it’s usually best to ensure that the foreground is sharp.

Some examples

A hat’s toss away: The closest pool was about 6 feet from my lens. I stopped down to f/20 (smaller than I generally like to go) and focused on the back of the pool on the left, about 10 feet away.

A baseball throw away: The little clump of wildflowers (lower right) was about 35 feet away and the trees started another 20 feet beyond that. With a focal length of 55mm, I dialed to f/11 and focused on the dead tree on the left, getting everything from the flowers to Half Dome sharp.

Why not just automatically set the aperture to f/22 and be done with it? Without delving too far into the physics of light and optics, let’s just say that there’s a not 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.

Believe it or not, with this foundation of understanding, 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.

In this image of a dogwood blossom in the rain, I positioned my camera to align Bridalveil Fall with the dogwood and used an extension tube to focus extremely close. The narrow depth of field caused by focusing so close turned Bridalveil Fall into a background blur (I used f/18 to the fall a little more recognizable), allowing viewers to feast their eyes on the dogwood’s and raindrop’s exquisite detail.

An extension tube on a macro lens at f/2.8 gave me depth of field measured in fractions of an inch. The gold color in the background is more poppies, but they’re far enough away that they blur into nothing but color. The extremely narrow depth of field also eliminated weeds and rocks that would have otherwise been a distraction.

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.

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Playing with Depth: A Gallery of Focus

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