Biomedical Engineering Reference
In-Depth Information
-8.85 D
-7.50 D
-13.67 D
F
A
= +6.79 D
-113.00 mm
I
SP
FP
O
-12.00 mm
-61.16 mm
-6.88 D
-16.35 D
-145.35 mm
Figure 8-11.
Accommodation required when a 7.50 D myopic eye (as measured in the
spectacle plane) views an object located 125.00 mm from the eye. The spectacle lens
forms a virtual image (I
SP
) 61.16 mm in front of the lens and 73.16 mm anterior to the
eye. This image serves as an object for the accommodative lens, which must image it
at the eye's far point (145.35 mm to the left of the eye's surface).
Before we leave this section, let's try a problem where we are given the refractive
error as measured in the spectacle, not corneal, plane.
An eye is corrected with a
−
7.50 D spectacle lens. How much accommodation is required when viewing an
object 12.50 cm from the eye while wearing the spectacle lens? Assume a vertex
distance of 12.0 mm.
First, we'll locate the image formed by the spectacle lens. As illustrated in
Figure 8-11, the object is located 113.00 mm (125.00
−
12.00 mm
=
113.00 mm) to
the left of the spectacle lens, giving us
L
′
=
+
F
L
(1000)(1.000)
⎥
⎦
L
′
=
+
(
−
7.50 D)
⎣
−
113.00 mm
′
=
−
L
16.35 D
n
′
′
=
L
′
l
(1000)(1.000)
′
=
l
−
16.35 D
′
= −
l
61.16 mm
