Hyperfocal Distance and Depth of Field
September 4, 2012
Several of my students at a recent weekend workshop were struggling with the concept of hyperfocal distance and depth of field. As always, when my students have a hard time understanding a topic, I assume that the fault lies with me: I simply didn't explain the subject well enough.
So here's a second try at clarifying this concept. First, a definition: depth of field refers to the zone of sharpness in an image from near to far. For example, you might say that the depth of field in a particular landscape print extends from six feet to infinity. All objects six feet away or farther appear to be sharp.
And now a second definition: the hyperfocal distance is the distance at which you focus, at a particular focal-length and aperture, to get the best possible depth of field. Depth of field extends from one-half of the hyperfocal distance to infinity. For example, the hyperfocal distance for a 24mm lens on a full-frame camera at f/22 is four feet four inches. If I focus the lens at that distance, depth of field will extend from two feet two inches to infinity.
That's the basic idea. Now let's dive in a little deeper.
You can actually only focus a lens at one particular distance. Only objects at the focused distance will truly be as sharp as the lens can deliver. Objects closer than the focused distance, as well as those farther away, will become progressively more blurry as the gap between them and the focused distance becomes larger and larger. But our eyes do not have infinite resolving power. So long as the width of the blurry edge of the object is narrower than a certain limit, we still see the object as sharp. One widely accepted standard for "sharp" is that the blurry edge cannot be more than 1/100th of an inch wide in the print. Since all prints require enlarging the tiny image captured by the sensor, the blurry edge on the sensor must be narrower still.
With that as background, you can now understand that all hyperfocal tables, and therefore all depth-of-field tables, are based on a series of assumptions, in particular on how big a print you want to make and how closely you want to view it. A particular image may have a depth of field from two feet to infinity when printed at 8x10 inches and viewed from a foot away. That same image may have a depth of field from four feet to infinity if you make a 30x40 inch print and view it from a foot away. View that 30x40 inch print from five feet away, however, and once again it has a depth of field from two feet to infinity. Our ability to resolve fine detail steadily declines as we get farther and farther from the subject. For example, you can easily read a book held at arm's length. Place it 10 feet away, and the task is impossible.
You should think of hyperfocal tables as optimistic. They provide a minimal level of sharpness that will be acceptable in magazine-size prints, but may not deliver the sharpness you need if you plan to make a big print. If your composition requires that you obtain the maximum depth-of-field possible, then by all means use them. If, on the other hand, you're shooting with a wide-angle lens and the closest object is 50 feet away, don't focus on the hyperfocal distance. Instead, focus on the closest object, which will essentially be at infinity with a wide-angle, stop down to f/8 or f/11 (usually your sharpest apertures), and fire away.
One other tip: if you need lots of depth of field, but not the very best the lens can deliver at a particular focal length and aperture, and if you want better sharpness throughout the image than a standard hyperfocal chart can deliver, use the hyperfocal distance for f/16, but shoot at f/22. For example, the hyperfocal distance with a 24mm lens at f/16 is six feet. I'll focus at six feet and expect depth of field from half that distance (three feet) to infinity, but with a higher standard of sharpness in the closest and farthest objects because I'm shooting at f/22 rather than f/16. (I'm setting aside the issue of loss of sharpness due to diffraction, a relatively minor problem which is a topic for another post.)
All of these calculations can seem rather befuddling when you're hypoxic and sleep-deprived. To double-check that you've got it right, shoot a test frame, enlarge it on your LCD by a factor of five or ten, and scroll back and forth across the image to be sure everything is sharp. The LCD on older cameras may look fuzzy everywhere at 10X, limiting the usefulness of this approach, but newer cameras give you a clear view even at 10X. When in doubt, compare sharpness at the focused distance (which should be razor-sharp if you're on a good tripod) with sharpness at the near and far limits of your composition.
Mastering depth of field and hyperfocal distance is a crucial part of the craft of landscape photography. And as I always remind my students, if you master the craft, the art will follow.
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