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Motion Picture News
Fundamentals of Light
(Continued from page 1484) This brings us to the consideration of the photographic lens and the principles which underlie its construction. By a lens is understood a piece of clear glass bounded by pol
Fig. 24 — Conjugate foci
ished curved surfaces. The various forms of simple lenses are divided into two general
Positive or Converging 1st. Double Convex. 2nd. Piano Convex. 3rd. Convexo-Concave. Negative or Diverging 1st. Double Concave. 2nd. Piano Concave. 3rd. Concavo-Convex.
The first are thickest in the center, while the second are thinnest in the center.
These simple forms may be made up of one single piece of glass or they may be composed of several cemented together, as will be seen later. Diagram 21 illustrates these forms of lenses.
All lenses, whether considered singly or in combination, have the following properties:
1. Principal axis.
2. Optical center.
3. Principal and conjugate foci.
4. Nodal points.
1st. Principal axis of a lens is a line passing through the thickest part of positive lenses and thinnest part of negative lenses, perpendicular to the surfaces of the lens, as in diagrams No. 22 and No. 23.
2nd. The optical center of a lens is the point from which focal measurements are
made. This does not refer to a photographic
objective which (in other than single view lenses) is a combination of lenses and quite another matter for the reason that a combination may have its optical center at a number of places according to the circumstances under which it is employed. The positive optical center of a lens is determined by its form as follows and shown in diagrams No. 22 and No. 23.
Draw two parallel radii AB and ab one from each center of curvature, and both inclined to principal axis; then connect the two points B and b at which they touch the curved surfaces of lens. The point 0, at which the line connecting B and b cuts the principal axis, is the optical center. In most cases the optical center is within the lens itself but in some cases, as with telephoto combinations and single meniscus lenses, it may be some distance outside the lens. Such an example is shown in Fig. 23.
3d. Conjugate foci. If a lens which has been carefully focused upon a distant object be then directed toward one comparatively near at hand, the nearer object will be found to be out of focus, necessitating the withdrawal of the ground glass from the lens before the image will assume its maximum sharpness. This establishes the fact that there exists a relation between the object that is focused, as regards its distance from the camera, and the focus of the lens. This relation is termed " conjugate foci." Foci is the plural of focus; conjugate means combined in pairs; kindred in meaning and origin. Conjugate foci are then the distances from the lens to the image and from the lens to the object. Hereafter we will speak of the distance between the lens and the object as the anterior or major conjugate, and that existing between the lens and the ground glass of the camera, as the posterior or minor conjugate focus. Parallel rays aa — that is, rays from
a great distance — falling upon a lens come to
a focus at f; but those from b, which may serve to represent any object ten or twenty yards distant, have their focus at c (Fig. 24). Then fo is the solar focus, bo and co are conjugate foci. The former of these is the anterior, and the latter the posterior conjugate. To facilitate reference, the lines indicating the conjugate foci are solid, while those relating to the solar focus are dotted. The points b and c are interchangeable; an object placed at either is sharp at the other.
Rule for Conjugate Foci. Now for every position of the object there is a certain position of the camera, and these two distances, the distance of the object from the lens and of the lens from the plate, are called conjugate foci.
A very simple mathematical rule connects the distance from lens to object (D) the distance from lens to plate (d) and the enlargement or reduction of the object (i.e., the number of times a given line in the object is larger or smaller in the image). Note the word line, because some prefer to calculate reduction and enlargement on the basis of area, which introduces different conditions.
Let F be the focal length of the lens and r the ratios of enlargement or reduction.
Then the distance d is equal to F plus F divided by r. Expressed more shortly: (Continued on page 1487)
Fig. 25 — Determination of Conjugate foci
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