Depth of field is the area within a photograph that is suitably sharp. It can extend broadly from the foreground to the background or encompass just a small sliver just an inch from front to back. Depth of field has a huge impact on the look of an image, so it’s important for a photographer to control it effectively. To see more of the image in focus—if the background is essential to provide context to the subject, for instance—the photographer changes camera settings to increase depth of field. To simplify the composition and minimize background detail—isolating a portrait subject from a busy background, for instance—the photographer makes changes to minimize depth of field. And changing depth of field requires changing more than just the aperture.
Several factors affect depth of field. First is lens choice. Put simply, for greater depth of field, use a wider lens. To minimize depth of field, choose a longer focal length. Technically, this is an optical illusion that works because as angle of view narrows, the scene is compressed and small changes to depth of field appear to have a greater impact. So the first step to controlling depth of field is to choose a lens that will help you achieve the effect you’re looking for: short lenses for more depth of field, long lenses for less.
The second factor that affects depth of field is the distance from the camera to the subject. More accurately, it’s the distance from the camera to the point of focus that really matters. If you’re far from the object you’re photographing and you focus closer to infinity, you’ll include a greater area in usable focus. But with the subject positioned very close to the camera, the same focal length and aperture produce shallower depth of field. You can see this in practice by looking at the barrel of a vintage lens.
Along with an adjustable aperture ring, manufacturers would print a depth of field guide on the lens barrel. Change the focus point and the guide would show how the area in usable focus changes. While such visual guides aren’t standard these days, lenses still work the same way. Stand closer to your subject and the depth of field shrinks. Stand farther away and focus nearer infinity and watch as depth of field expands.
Saving the best for last, the other major factor impacting depth of field—and the one we all think of first—is the aperture. Also known as ƒ-stop, the aperture measures the size of the opening in the lens and controls the amount of light allowed in. Remember that the smaller f-numbers correlate to larger openings, so that numbers such as ƒ/2 and ƒ/2.8 are wide apertures, while ƒ/22 and ƒ/32 are small apertures. Now remember this: the smaller the aperture, the greater the depth of field. The larger the aperture, the shallower the depth of field. So ƒ/2 is a great way to isolate the subject against an out-of-focus background, while ƒ/22 will help to make everything sharp from front to back.
Take a look at the examples shown above. With a wide-angle lens (35mm) there’s a lot in focus even at ƒ/4. But at the widest aperture—say ƒ/1.4—the background elements fall out of focus and better isolate the subject from the background. Yes, such isolation is standard operating procedure for portrait success, but if you want to show a sharp background for context, you’ll want to create greater depth of field. And now you know how to choose the appropriate lens, focus distance and aperture to make depth of field do your bidding.