Biomedical Engineering Reference
In-Depth Information
First let's calculate the power in the horizontal meridian
sin
2
θ
⎞
⎠
⎛
⎝
F
@180
=
F
1
+
2
n
sin
2
(15)
2(1.586)
⎛
⎝
⎞
⎠
F
@180
= −
8.00 D 1
+
F
@180
= −
8.17 D
Now, we'll calculate the induced cylinder at axis 180
Induced cylinder (axis 180)
=
F
(tan
2
θ
)
Induced cylinder
= −
8.00 D[tan
2
(15)]
Induced cylinder
= −
0.57 D
8.00 DS lens is in a frame with a pantoscopic tilt of 15 degrees, the
effective power experienced by the patient is
When a
−
180.
Note that pantoscopic tilt increases a minus lens's minus power. It is for this
reason that undercorrected myopic patients sometimes intentionally tilt their spec-
tacles to improve distance vision.
−
8.17
−
0.57
×
Curvature of Field
Not all points on the extended object in Figure 15-7 are the same distance from the
spherical converging lens. If you measure with a ruler, you'll see that the arrow's tip
F
F
′
Object
Plus lens
Petzval
surface
Figure 15-7.
The image plane for a flat object is curved, resulting in curvature of field.
The off-axis components of the object (the tip and base of the arrow) are further from
the lens than the on-axis components. Consequently, the image distance is less for
the tip and base.
