On Depth of Field

By Chris Walker

The phenomena of light coming into focus as it bends around a solid object – the basic principle behind depth-of-field – was first noted 7,000 years ago when the Chinese observed images of a solar eclipse being projected onto the ground as the celestial event came into focus by passing through the gaps of overlapping tree leaves. Since then, it has been used in the arts and sciences for everything from perspective studies during the renaissance to determining what class of planets may be circling distant stars.

By definition, depth-of-field is the amount of a scene's forward dimension that appears to be sharp in the final image that did not appear to be sharp through the lens. Those of us making fully-intentional photos can utilize this principle to dictate the degree to which each of our photographs is in focus. While the old adage says "the slower you shoot, the more you'll have in focus", a better axiom would explain that "the smaller your aperture, the more depth-of-field your picture will have". Although these statements are duly bound, it's the size of the camera's aperture – not the speed of the shutter – that determines depth-of-field.

In purely technical terms, acceptable focus is determined by the width of the "circles of confusion" in the out-of-focus areas of an image. As we stop down, the width of these circles decreases and the elements in previously-blurred areas become progressively clearer. So while a portrait of someone sitting on a park bench shot at f/2.8 (a very wide aperture) may only have the sitter's eyes in critical focus, the very same image shot at f/22 (a very narrow aperture) would yield an image with the entire body, and many background elements, acceptably sharp. In response to having changed the aperture to 22, the shutter speed must be slowed to compensate for the amount of light lost while passing through the diminished opening in the lens (hence the "the slower you shoot..." saying).

effects of aperture

The reduction of shutter speed is the most commonly noticed drawback resultant of using smaller apertures. While an image shot at 1/60th of a second at f/4 may give acceptable results on distant, less-dimensional scenes, for those scenes including significant foregrounds and distant backgrounds you may wish to use a smaller aperture, such as f/22, which will provide greater sharpness, overall, as well as a new shutter speed of ½ second – and the inability to hand-hold your camera.

A second, more significant, drawback comes through the image softness that can result from using smaller apertures. The sharpest f-stop on a lens is typically two to three stops down from the lens' maximum aperture; i.e., an f/2.8 lens will yield optimal sharpness at f/5.6 or f/8. Using smaller apertures will not ruin your images outright, but will cause diminished clarity on the micro level, blurring fine details within your subject.

Everyone has heard of pinhole cameras, those lensless wonders that create soft, ethereal photographs from even the most banal daily scenes. The aperture of a pinhole is frequently around f/256 – seven f-stops beyond the typical maximum aperture on most of our cameras! The effect of such an aperture is maximum depth-of-field, giving equal clarity to subjects from an inch away to infinity – all in the same frame. But maximum depth doesn't mean maximum sharpness, and this fading clarity doesn't magically hit at f/256 – it's a slow progression beginning around f/16. So for maximum sharpness, use apertures in the middle of your lens's range, reserving your use of f/22 and f/32 for those scenes that really call for substantial depth-of-field, when sacrificing that bit of sharpness will be worthwhile.

The best way to maximize sharpness is to use "hyperfocal distance". No matter what your aperture, the depth-of-field you have is dispersed through your image at a 1:2 ratio with a third of the added clarity falling in front of the focal point, and two thirds falling behind it. So let's say you're using a 35mm lens and are focused at infinity, with the immediate foreground about six feet away. If you'd like this foreground to be in focus, you can shoot at f/22 to gain that sharpness, but understand that only a third of your depth-of-field is falling in front of your focal point, so since you probably aren't shooting anything beyond infinity, you're wasting your depth-of-field. To rectify this, try shifting your focus to a nearer subject, around 10 feet, and open your aperture to something wider, such as f/11. In doing this, sharing your depth by choosing a hyperfocal point mid-way into your scene, the same amount of your image will be critically in focus at f/11, while focused at 10 feet, as it would have been at f/22 while focused at infinity! And, the added benefit is that you've made your image at a sharper aperture setting.

birch trees, fog

There are a few significant variables when working with depth-of-field, mostly in macro work and when changing focal lengths. In macro work, your depth-of-field has a 1:1 distribution, i.e., half in front of your point of focus, half in back. Next, you'll notice that telephoto lenses respond to aperture changes less than lenses in the normal and wide angle range do. Knowing this, you can intentionally switch to a telephoto and be assured that it will help isolate a subject, or switch to a wider lens and not only bring in a broader scene but also make it appear sharper, throughout.

tractor pair

This shift in depth values is also evident when changing formats. Anyone who's made the change from film to digital has noticed that a 24mm suddenly isn't as wide as it used to be, and that a 200mm now has a lot more reach than it once did. This is because when you change formats, the focal length needed to cover your media changes, too, which is partially why most digital cameras, having sensors that are smaller than a 35mm frame of film, require so little depth-of-field to yield similar depth to that of film cameras. Likewise, when picking up a 4x5" camera for the first time, you'll see that a 150mm is the format's "normal" lens and takes in the same approximate area as a 50mm does on a traditional 35mm film camera. However, even though the 150 is now normal, it still has the depth-of-field characteristics you'd expect from a telephoto used on a 35mm camera.

These notes and procedures are simply tools; they are not photography in and of itself. What's important is that you know how depth-of-field operates and that you actively put it to use in your own work.

As a general rule, greater depth-of-field tends to "flatten" an image, leaving spaces and objects looking two-dimensional. As much as that sounds like a bad thing, it's they way we see, as our eyes don't typically show us things in a manner that we think of as out of focus. We look at a table, it's in focus. We look out the window beyond, and it, too, appears to be in focus because our eyes adapt so quickly that we rarely notice the shift.

In contrast to this, selecting a narrow focal point by limiting your depth-of-field can firmly direct your viewers' attention to an individual detail. In example, the image shot at f/4 of the tractor really makes you concentrate on the old headlight, while the image shot at f/32 presents a general scene, allowing the viewer to look through it and see different elements of the image represented more equally than in the f/4 version.

With the mallard decoy, I thought the image showing more depth-of-field would be my pick, but the way the bird's bill blends into the workshop tools in the background is really distracting, so I've ended up preferring the shallow depth image over the other.

decoy pair

Sometimes, whether an image is fully in focus or not isn't so open to discussion – I can hardly imagine what a similarly done image of the garlic cloves with the old flour mill in the background would look like if shot wide open – regardless, this isn't a wrong or right issue – it's reliant on each photographer's individual taste.

heirloom garli

Occasionally we find a scene that's already two-dimensional, such as this tool wall in an abandoned grain elevator. Do we even need to consider shooting a scene such as this at f/22? Probably not, at least in small formats, but shooting it wide open would produce an image lacking crispness, so I used an aperture in the middle of the available scale, making a negative that was optimally sharp, front to back, with no compromises.

tool wall brandon handlin

Finally, sometimes we don't want a lot of depth of field. A few years ago, for my county fair project, I got tired of the scattered information in my images' backgrounds competing with my subjects for attention, so I changed formats – abandoning my usual 35mm and taking up an 8x10" view camera to do the portraits. I still use a wide-angle lens, but on an 810 my wide angle of choice is a 180mm; although it's the equivalent of a 28mm in 35 terms, it's still a telephoto lens, so I'm able to get the angle of view I desire without the carnival lights drowning our my sitters.