5 ×7-inch print viewed at arm’s length, might appear blurry when blown up to 11 ×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 dif-
ferences 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 (see Figure 5.9). In practice, the discs in many 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.
Chapter 5 ■Mastering Autofocus Options 141
Figure 5.9
Only the blos-
soms in the
foreground are
in focus—the
area behind
them appears
blurry because
the depth-of-
field is limited.