Figure 2-44. Focal lengths of convergent lens.

TM 9-258

object. If a sheet of ground glass or thin paper is placed

focus (fig 2-44). The point of principal focus of a lens

across the back of a camera or held in back of a lens at

does not vary, it always remains at the same distance

a distance that will permit the lens to focus the images of

from the lens. The optical center of a lens is a point at

objects on the glass or paper, images of all distant

the center of the thickest part of a converging lens or the

objects may be brought into sharp focus at one time

thinnest part of a divergent lens. It lies on a straight line

regardless of their distance providing the distance is

connecting all the principal focal points of the lens (fig 2-

great -and rays of light are almost parallel. An object

45). Rays of light pass through the optical center without

very close to the lens will be out of focus. The ground

deviation. The distance from the point of principal focus

glass or paper will have to be moved backward to bring

to the optical center of a convergent lens is the focal

such an object into sharp focus; then the more distant

length of the lens. This is a fundamental concept which

objects will be out of focus.

must be kept in mind throughout to understand optical

c. Point of Principal Focus. The focal point where

diagrams. The thicker the lens in relation to its size or

the steeper the curves of the lens, the shorter its focal

almost parallel rays from a very distant object intersect is

length will be.

called the point of principal

Figure 2-44. Focal lengths of convergent lens.

d. Focal Plane. The focal plane of the lens (fig 2-

44) is a thin flat area at right angles to the central axis of

the lens in all directions from the focal point. It is the

area that is occupied by the ground glass or paper which

might be used to make the image visible. In a camera,

the film is placed in the focal plane of the lens. The focal

point (fig 2-46) is the image of a single point of light on

the object; the focal plane is the combined image of all

the points of light on the object. The principal focal plane

of the lens is the focal plane passing through the point of

principal focus (fig 2-44). In a camera, when taking a

photograph of very distant objects, the film is placed in

the principal focal plane of the lens.

Figure 2-45. Optical center of lens.

e. To Determine Focal Length of Convergent

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