
MATRIX OPTICS 79
and the front vertex of the second lens should be placed at
zV2 = 3.6 mm +20 mm = 23.6 mm. (3.84)
Likewise, all the other points can be found in the global coordinates by adding appropriate
distances.
3.4.4 Telescopes
In the example of the compound lens in Section 3.4.1, we noted one special case which is of
great practical interest. In Equation 3.65,
1
f
=
1
f
2
+
1
f
1
−
z
12
f
1
f
2
,
if the separation of the two lenses (measured from the back principal plane of the rst to the
front principal plane of the second) is equal to the sum of their focal lengths, then the optical
power is zero, and the compound lens is said to be afocal. A telescope is