Understanding Depth of Field

Apr 7, 2011 by

Understanding Depth of Field

One of the more important concepts to come out of having a basic understanding of aperture and shutter speed is depth of field (DOF). Depth of field refers to that portion of your photo that is acceptably in focus, and mastery of depth of field is one of the two main factors (the other being lighting) that can transform an image into a beautiful piece of art.

To get an idea of how DOF operates, imagine holding a piece of paper at arms length in front of you so that it is facing you. This paper is a essentially a two-dimensional plane, just like the plane of focus in your camera is. Focus your eyes only on that paper, and you’ll notice peripherally that your arms in front of you are out of focus, and so is the background. Human eyes have only a narrow plane in which things are in focus as well, but where they differ from a camera is that they can rapidly change focus so that within a second, you can make your arm or the background in focus (and, of course, make the paper out of focus).

When you snap a photo, whatever is in focus on the image is the only thing the human eye can see. If you had a camera up to your eye as you were holding the paper in front of you and focused narrowly on the piece of paper, the resulting image would show your arm and background as out of focus, and no amount of focusing by your eye on that photo can change that. As such, it is important that when you take a photo, you have everything that you want to be in focus before you actually press the shutter button and take the photo.

One of the major things that dictates DOF is aperture. The larger the aperture (and smaller the f-stop), the less of the image that will be acceptably in focus (Figure 1), whereas the smaller the aperture (and larger the f-stop), the more of the image that will be acceptably in focus (Figure 2). Of course, it is never quite that simple, as shutter speed changes with every change of your aperture. As such, it is frequently necessary to make compromises between aperture and shutter speed so that you get an acceptable amount of DOF, while at the same time not having too fast or too slow a shutter speed (Figure 3).

Depth of field also is influenced by focal length and shooting distance. Focal length is expressed in millimeters, and governs how large an image is in your viewfinder (and consequently in your photograph). The higher the number, the more magnified your subject gets. In terms of DOF, longer focal lengths result in some reduction of DOF relative to shorter focal lengths, but where it is most noticeable is with the distribution of DOF. At shorter focal lengths, such as on wide-angle lenses, the DOF is distributed more in front of the subject than behind it, but on longer lenses, such as on telephoto lenses, DOF is distributed more equally between front and back. Shooting distance also influences DOF. All other things being equal, the greater the distance from your subject, the deeper the DOF.

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