Biophotonics_Concepts_to_Applications

number, which is called thefield of view(FOV) or thefield diameter, allows the
viewer to estimate the size of the object being examined. Thefield of view can be
calculated from a number that is listed on the microscope eyepiece and from the
magnification of the objective. A typical example of two numbers on the eyepiece
might be 10X/22. In this case, 10X is the magnification of the eyepiece and 22 is
called thefield-of-view number, or simply thefield number(FN), which is given in
mm. Typically thefield number increases as the magnification of the eyepiece
decreases. For example, a 5X eyepiece might have a FN of 26 mm, for a 10X
eyepiece FN may be 22 mm, and for a 30X eyepiece the FN might decrease to
7 mm. To calculate the diameter of thefield of view (measured in mm), one divides
the FN on the eyepiece by the magnification Mobjof the objective lens, as given by

FOV¼ FN/Mobj ð 8 : 2 Þ

By knowing the diameter of the FOV and counting how many times the object
fits across this FOV, then the following equation can be used to calculate the object
size:

Object size¼

Diameter of FOV

Number of times the object fits across the FOV

ð 8 : 3 Þ

Example 8.3Consider an objective lens that has a 10X magnification and

which is used with an eyepiece offield number 22.

(a) What is thefield of view?

(b) Suppose an object that is viewed by this eyepiece and objective combi-

nationfits across the FOV 2.75 times. What is the size of the object?

Solution:

(a) From Eq. (8.2) the FOV in mm is: FOV = FN/M = (22 mm)/

10 = 2.2 mm

(b) From Eq. (8.3) the object size is FOV/2.75 = (2.2 mm)/2.75 = 0.80 mm

8.1.5 Depth of Field

The parameterdepth offield(DOF) refers to the thickness of the plane of focus.
That is, the depth offield indicates the range of depth over which the nearest and
farthest object planes of a viewed specimen are in acceptable focus simultaneously.

242 8 Microscopy