Chapter 7 Optical Characteristics
2 Microscope Terminology
76
(5) Field Number of the Eyepiece
The diameter of the opening of the fieldstop inside the eyepiece measured in mm.
When an eyepiece has an indication of “10x / 20”, it means that the magnification is
10x and the field number is 20 for that eyepiece.
(6) Real Viewfield
The diameter in mm of the field of view observable through the eyepiece.
Real viewfield = field number of eyepiece / magnification of objective
(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×
λ
＋
..M
n
ΑΝ××7
×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.75 is indicated forλ
= 0.55µm.)
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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