Adding Circles of Confusion

But there are other factors in play, as well. You know that increased depth-of-field brings more of your subject into focus. But more depth-of-field also makes autofocusing (or manual focusing) more difficult because the contrast is lower between objects at different distances. So, autofocus with a 200mm lens (or zoom setting) may be easier than at a 28mm focal length (or zoom setting) because the longer lens has less apparent depth-of-field. By the same token, a lens with a maximum aperture of f/1.8 will be easier to autofocus (or manually focus) than one of the same focal length with an f/4 maximum aperture, because the f/4 lens has more depth-of-field and a dimmer view. That's why lenses with a maximum aperture smaller than f/5.6 can give your T2i's autofocus system fits.

To make things even more complicated, many subjects aren't polite enough to remain still. They move around in the frame, so that even if the T2i is sharply focused on your main subject, it may change position and require refocusing. An intervening subject may pop into the frame and pass between you and the subject you meant to photograph. You (or the T2i) have to decide whether to lock focus on this new subject, or remain focused on the original subject. Finally, there are some kinds of subjects that are difficult to bring into sharp focus because they lack enough contrast to allow the T2i's AF system (or our eyes) to lock in. Blank walls, a clear blue sky, or other subject matter may make focusing difficult.

If you find all these focus factors confusing, you're on the right track. Focus is, in fact, measured using something called a circle of confusion. An ideal image consists of zillions of tiny little points, which, like all points, theoretically have no height or width. There is perfect contrast between the point and its surroundings. You can think of each point as a pinpoint of light in a darkened room. When a given point is out of focus, its edges decrease in contrast and it changes from a perfect point to a tiny disc with blurry edges (remember, blur is the lack of contrast between boundaries in an image). (See Figure 5.7.)

Figure 5.7

When a pinpoint of light (left) goes out of focus, its blurry edges form a circle of confusion (center and right).

Figure 5.7

When a pinpoint of light (left) goes out of focus, its blurry edges form a circle of confusion (center and right).

If this blurry disc—the circle of confusion—is small enough, our eye still perceives it as a point. It's only when the disc grows large enough that we can see it as a blur rather than a sharp point that a given point is viewed as out of focus. You can see, then, that enlarging an image, either by displaying it larger on your computer monitor or by making a large print, also enlarges the size of each circle of confusion. Moving closer to the image does the same thing. So, parts of an image that may look perfectly sharp in a 5 X 7-inch print viewed at arm's length, might appear blurry when blown up to 11 X 14 and examined at the same distance. Take a few steps back, however, and it may look sharp again.

To a lesser extent, the viewer also affects the apparent size of these circles of confusion. Some people see details better at a given distance and may perceive smaller circles of confusion than someone standing next to them. For the most part, however, such differences are small. Truly blurry images will look blurry to just about everyone under the same conditions.

Technically, there is just one plane within your picture area, parallel to the back of the camera (or sensor, in the case of a digital camera), that is in sharp focus. That's the plane in which the points of the image are rendered as precise points. At every other plane in front of or behind the focus plane, the points show up as discs that range from slightly blurry to extremely blurry, as you can see in Figure 5.8. In practice, the discs in many

The flower is in focus, but the area behind it appears blurry because the depth-of-field is limited.

of these planes will still be so small that we see them as points, and that's where we get depth-of-field. Depth-of-field is just the range of planes that include discs that we perceive as points rather than blurred splotches. The size of this range increases as the aperture is reduced in size and is allocated roughly one-third in front of the plane of sharpest focus, and two-thirds behind it. The range of sharp focus is always greater behind your subject than in front of it.

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