Nikon eclipse e100 Instructions Manual page 36

(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 / 18", it means that the magnification is 10x and
the field number is 18 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 ( m) =
Above is an approximation assuming the resolving power of an eye as 2 minutes.
is the wavelength of light used. (The depth of focus in the table on P. 33 is indicated for
= 0.55 m.)
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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