Figure 5-15. Objective diameter limits field in Galilean system

TM 9-258

b. Uses. This system is quite inexpensive

is used frequently as a reduction view finder on cameras

by reversing it with the positive element used as the

because of its simplicity and is used in field glasses. It is

eyelens and the negative as the objective. Because of its

used in opera glasses because of its very short length

low power limited field, and lack of real image, plane, it

and light weight. Note that is provides an erect image

has no application in military instruments in this country.

without lens or prismatic erectors in a system shorter

than a modern astronomical system of equal power. It

Section IV. FUNCTIONS OF COLLECTIVE LENSES AND EYEPIECES

rays would miss the erectors without it and leave the

5-12.

Functions of Collective or Field Lenses.

outer portions of the field with less illumination than the

a. Primary Functions. This element (usually

central portion. The clear aperture required in the lenses

plano-convex) may be placed in a system with the plano

in the erecting system is reduced so that smaller lenses

side coincident with the image plane (focal plane) of the

can be used or the collective lens can be considered to

objective. If the principal plane of this lens lies in the

increase the field, if the erectors are of sufficient size and

image plane of the objective, this lens does not alter the

the eyepiece has sufficient apparent field.

power of the instrument but reduces the power if placed

closer to objective (fig 5-16).

The collective lens

provides balanced illumination of the entire field. Rim

5-8