Biomedical Engineering Reference
In-Depth Information
We can also specify lens's power with respect to its front surface (see
f
n
in
Fig. 6-2). Referred to as the
front vertex
or
neutralizing power
and abbreviated
F
n
, it is given by following formula:
F
2
F
n
=
+
F
1
t
⎛
⎞
⎠
1
−
F
2
⎝
In the procedure called hand neutralization, trial lenses of known power are held
against the front surface of a lens of unknown power to neutralize the movement
seen through the unknown lens as it is moved. When the movement has been neu-
tralized, the power of the trial lens is equal and opposite the unknown lens's front
vertex power. Front vertex power may also be used to specify the power of bifocal
adds (see Chapter 8 and Fig. 11-9).
Let's take this opportunity to point out a source of confusion that is common when
first learning this material. Students sometimes confuse
F
1
and
F
2
with
F
v
and
F
n
.
Let's be sure we understand that
F
1
and
F
2
refer to the powers of the front and
back surfaces of the lens, while
F
v
and
F
n
give the power of the lens itself (taking
into account the lens's two surfaces).
EQUIVALENT LENSES
Equivalent Power
As we discussed earlier in this chapter, the equivalent power of a lens,
F
e
, specifies
the lens's power with respect to its principal planes. It can be calculated using the
following formula:
t
⎞
⎠
⎛
F
e
F
1
+
F
2
−
F
1
F
2
=
⎝
Equivalent power can be used in the vergence equation to determine the location of
an image (or object) relative to the principal planes.
Locating the Principal Planes
As the shape of a lens changes, the positions of its principal planes change even
when the equivalent power of the lens remains constant. This can be seen in
Figure 6-3, which shows how the principal planes for plus and negative lenses move
as the front surface becomes more convex. Note that when the front surface is very
convex, principal planes can be located outside of the lens itself.
