Biomedical Engineering Reference
In-Depth Information
For a 5.00-mm-diameter pupil, the peaks of the Airy's disks must be separated by
at least 0.47 minutes of arc. If we repeat this calculation for a 2.00-mm-diameter
pupil, we find that the Airy's disks must be separated by at least 1.16 minutes of arc.
As the pupil becomes smaller, the diffraction patterns become larger; consequently
the patterns (and point sources) must be farther separated from each other in order
to be resolved.
3
DEPTH OF FIELD AND DEPTH OF FOCUS
Due to what is called
depth of field
, an object, such as the optotype
E
, does not
need to be focused on the retina for us to resolve its details. As long as the retinal
blur circles are not too large, it can be resolved. (If the blur circles are too large,
they will overlap each other, preventing resolution.)
Take a look at Figure 11-7, which shows an eye with 3.00 D of uncorrected
myopia that is refractive in origin. Let's assume the eye manifests absolute pres-
byopia, meaning that it has no ability to accommodate. From Chapter 8, we know
that an object located at a distance of 33.00 cm is conjugate with the retina. This
eye's depth of field, however, permits objects as far away as 40.00 cm or as close as
28.57 cm to be resolved.
1.00 D or 11.43 cm
1.00 D or 0.37 cm
-40.0 cm
or
-2.50 D
-28.57 cm
or
-3.50 D
+22.03 cm
or
+60.50 D
+22.40 cm
or
+59.50 D
-33.33 cm
or
-3.00 D
+63.00 D
Depth of focus
Depth of field
Figure 11-7.
The depth of field is conjugate to the depth of focus. Although the depth
of field and depth of focus are dioptrically equal (1.00 D), they are not linearly equal
(11.43 cm vs. 0.37 mm). (The 3.00 D myopic eye in this figure is uncorrected and
unable to accommodate.)
3. The packing density of retinal photoreceptors limits our ability to resolve detail less than
about 0.50 minarc. In the case of a 5.00-mm-diameter pupil, resolution is not likely to be limited
by diffraction but by the density of the photoreceptors and optical aberrations (Chapter 15). As the
pupil diameter decreases to about 2.00 to 3.00 mm, diffraction becomes the limiting factor.
