length have a greater NA (i.e., a greater light-gathering ability) and yield afiner
resolution of the sample.
Example 8.1Consider a series of objective lenses in which the half-angle of
the captured light cone varies from 7° to 60°. What is the range of numerical
apertures if the immersion medium is air?
Solution: From Eq. (8.1) with n = 1.00, as the angleαincreases from 7° to
60° (as the light cones grow larger) the numerical aperture increases from
0.12 to 0.87.
Example 8.2Consider a series of objective lenses in which the half-angle of
the captured light cone varies from 7° to 60°. What is the range of numerical
apertures if the immersion medium is oil with n = 1.51?
Solution: From Eq. (8.1) with n = 1.51, as the angleαincreases from 7° to
60° (as the light cones grow larger) the numerical aperture increases from
0.18 to 1.31.
Note: Objective lenses with a magnification range between 60x and 100x
typically are designed for use with immersion oil. In practice, most oil
immersion objectives have a maximum numerical aperture of 1.4, with the
most common numerical apertures ranging from 1.00 to 1.35.
8.1.4 Field of View
When looking into a microscope, it is useful to know the diameter of the viewed
field in millimeters measured at the intermediate image plane. Knowing this
D
D
D
Specimens
α = 8°
α = 20°
α = 60°
For NA = sin α
(a) NA = 0.14 for α = 8°
(b) NA = 0.34 for α = 20°
Objective lens (c) NA = 0.87 for α = 60°
Light cones
Focal
length f
(a)(b)
(c)
Fig. 8.5 The NA increases for short objective focal lenses
8.1 Concepts and Principles of Microscopy 241