Biomedical Engineering Reference
In-Depth Information
By convention, depth of field is given in diopters.
In the current example
(Figure 11-7), the dioptric equivalent of 40.00 cm is 2.50 D (i.e., 100/40.00 cm
=
2.50 D) and the dioptric equivalent of 28.57 cm is 3.50 D (i.e., 100/28.57 cm
=
3.50 D cm), making the depth of field equal to 1.00 D. This corresponds to a linear
range of 11.43 cm. The depth of field is always dioptrically centered at a point con-
jugate with the retina, which in this case is 3.00 D (equivalent to 33.33 cm). Another
way of saying this is that the depth of focus is
0.50 D centered at 3.00 D.
Figure 11-7 also shows there is a range centered on the retina—the
depth of
focus
—that is conjugate with the depth of field. Like the depth of field, the depth
of focus is 1.00 D, but this corresponds to a linear range of only 0.37 mm. You must
be wondering, “How did we arrive at this”? Here's how. Keeping in mind what we
learned in Chapter 7 and that this myopic eye has a power of
±
63.00 D, we can
locate the image produced when the object distance is 40.00 cm as follows:
+
′
=
L
L
+
F
1000
400.0 mm
−
(1000)(1.333)
⎛
⎝
⎞
⎠
=
+
63.00 D
′
l
′
=
l
+
22.03 mm
For an object distance of 28.57 cm, we have
′
=
L
L
+
F
(1000)(1.333)
1000
285.7 mm
−
⎛
⎝
⎞
⎠
=
+
63.00 D
′
l
′
=
l
22.40 mm
The difference between these two distances (0.37 mm) is the linear range that
corresponds to the depth of focus.
Let's summarize what we've discussed so far. There is a range—the depth of
field—over which an object can be resolved. It is centered dioptrically at the
distance the eye is focused. Conjugate to the depth of field is the depth of focus,
which is dioptrically centered on the retina. When in dioptric units, the depth of
field is equal to the depth of focus.
The extent of the depth of field (and depth of focus) depends primarily on the pupil
diameter. A small pupil decreases the size of the blur circles and thereby increases
the depth of field. (If the pupil becomes too small, diffraction and reduced retinal
illumination may limit visual resolution, but we'll ignore these factors for now.)
Let's look at an example.
An emmetropic patient with absolute presbyopia wears
a near correction of
2.50 D. His total depth of focus is 0.50 D. What is his linear
range of near vision when he wears his presbyopic correction? What would be
his linear range of clear vision if he wore a near correction of
+
+
1.00 D?
