Page 326 - Modern Optical Engineering The Design of Optical Systems

Optical System Layout 305

The magnifying power is the ratio between these two angles
′ h (f s′) ( 10 in)
MP
fs′ h
10 in 10 in
(13.10)
f s′
Thus we find that the magnification produced by a simple micro-
scope depends not only on its focal length but on the focus position
chosen. If one adjusts the object distance so that the image is at infinity
(i.e., s f and s′ #) and can be viewed with a relaxed eye, then the
magnification becomes simply
10 in
MP (13.10a)
f
If the focus is set so that the image appears to be 10 in away (i.e., s′
10 in) then
10 in
MP 1 (13.10b)
f
The value of MP given by Eq. 13.10a is conventionally used to express
the power of magnifiers, eyepieces, and even compound microscopes.
The preceding assumed that the eye was located at the lens. If the
image is not located at infinity, the magnifying power will be reduced
as the eye is moved away from the lens. If R is the lens-to-eye distance,
the magnification becomes
10 (f s′)
MP (13.10c)
f (R s′)
Note that if the dimensions are in millimeters, the constant 10 becomes
254, or 250.

13.5 The Compound Microscope
As illustrated in Fig. 13.12, a compound microscope consists of an
objective lens and an eyelens. The objective lens produces a real

Figure 13.12 The compound microscope.