Figure 2-46. Focal length of divergent lens.

TM 9-258

Lens. A rough determination of the focal length of a

planes resulting from the nearness of the object to the

convergent lens can be secured by holding the lens to

lens are located on the side of the divergent lens toward

focus the image of some distant object on a sheet of

the object or light source. The point of principal focus

paper or ground glass. When the image is clear and

and other focal points are located where the emergent

sharp, this will indicate that the point of principal focus

rays would intersect on the axis if they were extended

has been reached. Measure the distance from the

backward as imaginary lines toward the side of the lens

image to the optical center of the lens.

on which the light strikes (fig 2-46). The distance from

f. Focal Length of Divergent Lens. The point of

the point of principal focus to the optical center of the

lens is the focal length.

principal focus and any focal points and focal

Figure 2-46. Focal length of divergent lens.

telescope will produce an image, the apparent size of

2-23. Magnification and Reduction.

which is eight times as high and eight times as wide as

a. Magnification is the apparent enlargement of the

the image when viewed by the unaided eye.

image of the object to the eye of the observer. Common

d. Military targets can often be seen better with 10-

examples are the reading glass and microscope. Inafire-

power magnification than with 20-power, due to the

controlinstrument, magnification, by marking targets

change in the field of vision which accompanies a

appear larger, makes them appear closer.

change in power and due to varying conditions of light.

b. Optical reduction is the apparent reduction in the

For this reason, some fire-control instruments are fitted

size of the image of the object to the observer. A

with means for changing the power of magnification.

negative or divergent lens placed in the floor of an

Certain instruments provide no magnification; their

airplane to permit observation functions on this principle.

primary purpose is to place a reticle in the field of vision.

It makes objects appear smaller than when viewed with

the unaided eye but has the advantage of increasing the

2-24. Image Formation by Convergent Lens.

field of view. A much larger area may be scanned

a. Types of Images. Images which are formed by

through the relatively small opening than would be

convergent lenses may be either real or virtual images.

possible if the lens were not used.

c. The degree of magnification of an optical

They may be larger or smaller than the object, depending

upon the distance of the object from the lens.

instrument is expressed as the power of the instrument,

Perfect Lens Assumed. Multiple or com-

for example, 8-power. An 8 power (8X)

2-32