VII Optical Characteristics
43
VII
(7) Depth of Focus
The depth (thickness) of the specimen image in focus, extending above and
below the focused image plane. The larger the N.A. of the objective, the
shallower the depth of focus.
Depth of focus [µm] ＝
2
..2
n
ΑΝ×
＋
..M7
n
ΑΝ××
×1000
Above is an approximation assuming the resolving power of an eye as 2 minutes.
λ is the wavelength of light used. (The depth of focus in the table on P. 42 is indicated for
λ = 0.55 µm.)
n is the refractive index of a medium between the objective lens and the specimen or
condenser. (n = 1, when the medium is air, and n = about 1.5 when the medium is oil.)
M is the total magnification (= the individual magnifying power of the objective x that of
the eyepiece).
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