DETERMINING THE SIZE OF THE
FIELD OF VIEW
The field of view is the circle of light that you see when you look through the eyepiece. If the eyepiece, tube, lens, slide, stage opening, diaphragm opening and light source are lined up correctly, a bright white circle should be visible.
If you know the size of the field of view, then you can estimate the size of what you are looking at. If you know the diameter of the field of view, then you can determine the length or width of the specimen. When you know the length and width, you can determine the area.
*The field of view can be measured with ruled micrometers. An ocular micrometer is a glass disk with spaced lines etched at regular, but unknown intervals. The field of view is determined when the ocular intervals are compared to those of known units on a stage micrometer.
This procedure (as follows) is courtesy of Vodopich and Moore, Biology
Laboratory Manual, 1992:
DETERMINE SIZE OF FIELD OF VIEW USING OCULAR AND STAGE MICROMETERS
1. Rotate the ocular until the lines of the ocular
micrometer parallel those of the stage micrometer (Fig. 1-3).
2. Align lines at the left edges of the two
micrometers by moving the stage micrometer (Fig. 1-3).
3. Count how many spaces on the stage micrometer fit
precisely in a given number of spaces on the ocular micrometer. For example, 10
spaces on the ocular micrometer may be exactly the same width as 8 spaces on
the stage micrometer.
4. Calculate the distance in mm between lines of the
ocular micrometer. Since the smallest space on a stage micrometer = 0.01 mm,
spaces on the ocular = N spaces on the
“stage” x 0 01 mm
space on the ocular = (N spaces on the
stage x 0.01 mm)/10
example, if 10 spaces on the ocular equal 8 spaces on the stage, then 1 ocular
space = (8 x 0.01 mm)/10 = 0.008mm = 8 mm
a specimen spans 8 spaces on your ocular micrometer with that objective in
place, that specimen is 64 mm long.
5. Calibrate the ocular micrometer for each objective
on your microscope. Record in Table 1-1 the diameter of the field of view for
6. Use this information to determine the area of the
circular field of view with the following formula:
of circle = p
* The field of view can also be determined by an alternate procedure-- using a transparent metric ruler.
Complete this table, by using the procedure that follows.
OBJECTIVE EYEPIECE TOTAL FIELD OF VIEW FIELD OF VIEW
MAGNIFICATION MAGNIFICATION MAGNIFICATION DIAMETER (MM) AREA (MM2)
1. FOCUS A METRIC RULER UNDER THE LOW POWER OBJECTIVE LENS.
2. USE THE RULER TO DETERMINE THE DIAMETER OF THE FIELD OF VIEW.
PLACE A LINE ON THE RULER AT THE FAR LEFT OF THE WIDEST PART OF THE BRIGHT CIRCLE.
COUNT THE NUMBER OF LINES IT TAKES TO CROSS THE CIRCLE.
*IT MIGHT BE NECESSARY TO ROUND OFF.
EACH LINE REPRESENTS 1 MILLIMETER.
RECORD THE DIAMETER IN THE TABLE.
3. THE RULER CANNOT BE USED TO FIND THE DIAMETER OF THE MEDIUM AND HIGH POWER MAGNIFICATIONS, SO USE THIS FORMULA:
*(DIAMETER LOW POWER FOV) X (MAGNIFICATION OF LOW POWER) DIVIDED BY (THE MAGNIFICATION OF THE MEDIUM POWER) = THE DIAMETER OF THE MEDIUM POWER. RECORD THE DIAMETER OF THE MEDIUM POWER FIELD OF VIEW IN THE TABLE.
*REPEAT THE FORMULA TO DETERMINE THE HIGH POWER FIELD OF VIEW DIAMETER.
4. FIND THE AREAS OF THE 3 FIELDS OF VIEW BY USING THE FORMULA FOR THE AREA OF A CIRCLE:
area = p(radius2) REMEMBER: p = 3.14 and the radius is 1/2 of the diameter