Biomedical Engineering Reference
In-Depth Information
11
Depth of Field
An object that is conjugate with the retina is by definition focused on the retina. If
the object is a point, then the image is a point. Figure 11-1A shows a myopic eye
that has no ability to accommodate (i.e., the eye of a patient with
absolute pres-
byopia
). Point
Y
is imaged onto the retina at
Y
. For an object that is farther from
the eye—say at
X
—the image is focused anterior to the retina at
X
′
. This object
produces a blurred image on the retina that is called a
blur circle
.
A similar effect
occurs for an object at
Z
: the image is focused at
Z
′
and a blur circle is formed on
the retina. Figure 11-1B shows how these blur circles may appear to the observer.
The size of the blur circle depends on both the amount of defocus and the diam-
eter of the pupil. As illustrated in Figure 11-2, as the focused image moves closer to
the retina, the diameter of the blur circle decreases. Reducing the size of the pupil
blocks peripheral rays, also reducing the size of the blur circle (Fig. 11-3).
′
BLUR CIRCLES, VISUAL ACUITY, AND PINHOLES
Visual acuity is measured by asking a patient to read optotypes, such as the letter
E
, presented on an eye chart. Optotypes are extended sources, meaning that they
are composed of an infinite number of point sources. Figure 11-4A shows how the
optotype
E
can thus be conceptualized.
Consider a patient with 1.00 D of uncorrected myopia who observes the opto-
type
E
on a visual acuity chart that is located 20 ft away (optical infinity). Since the
eye is not focused for this distance, each of the point sources forms a blur circle on
the retina—similar to what is depicted in Figure 11-4B—and the
E
appears blurry
to the patient. Suppose we ask the myopic patient to view the eye chart through a
small aperture (approximately 1 mm in diameter) referred to as a
pinhole
, which is
smaller than her pupil
.
1
As can be seen in Figure11-4C, this causes the diameters
1. When a patient looks through an aperture that has a diameter less than her pupil's, the aperture is
sometimes referred to as an artificial pupil.
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