Moving Picture World (Aug 1917)

1074 THE MOVING PICTURE WORLD August 18, 1917 . g> g« e6 ^ e?° gj» g:« ec» g g° g> g> c» ST» g c: « • ♦ js q» ^ *s> »js «;s °:S~^ ^i . ■ — : — : ~ . — ~ — .. Motion Picture Photography Conducted by CARL LOUIS GREGORY, F. R. P. S. ft I j ■ — ■ J • 6t» g£ g£« g> g» g> ^ifLS g° g" g° ^'El^ ^ ' * * ^^<^°^ °^^ ^ '^ °S OgjgS g *S g jg g ! Inquiries. QUESTIONS in cinematography addressed to this department will receive carbon copy of the department's reply by mail when four cents in stamps are inclosed. Special replies by mail on matters which cannot be replied to in this department, $1. Manufacturers' Notice. It Is an established rule of this department that no apparatus or other goods will be endorsed or recommended editorially until the excellence of such articles has been demonstrated to its editor. Optical Terms (Continued). (Adopted for cinematographers from material furnished by the Bausch & Lomb Optical Company.) THAT the amount of indistinctness permissible on the picture is susceptible of numerical expression is easily seen from the following : If an object at a given distance is in sharp focus, the light issuing from a point of that object is converged to a point on the plate. Light issuing from a point in the original object will also be converged to a point, but not on the plate, the cone of light showing in either case a circular patch of light on the plate. This circle of light is known as the "circle of confusion." Its diameter can be used to express the amount of indistinctness existing in a picture. If the circle of confusion is not greater than 1/10 mm. or 1/250 inch, it would appear as a point to an eye 10 inches away, hence, an object no point of which is imaged by a circle larger than 1/10 mm. would appear sharp. No matter what their type of construction may be, all lenses of the same equivalent focus and the same relative aperture require the same exposure — that is, have, the same speed, other conditions being equal. They will also have the same depth. The depth of focus decreases : 1. With increase of focal length. 2. With increase of relative aperture (speed). 3. With increasing nearness of object. Of the two lenses of the same equivalent focus, the one with the lower relative aperture (speed) has the greater depth of field. On the other hand : if the focal length of the lens is very short, a speed as high as F :4.5 will allow bringing every object from 10 feet to infinity to a sharp focus, while a studio lens of long focus and the same speed may not even image an object of the depth of a head sharply within the range of the length of a studio. Speed, great focal length and depth of focus cannot be combined in the same lens. This is an unalterable law of optics. If speed be the most desirable quality, depth of focus must be sacrificed ; if depth of focus, speed. This does not detract from the value of fast lenses, because with a given lens the depth of focus can be increased by diaphragming down the lens, which means reduction of speed. If a short exposure demands the use of the lens wide open, one must not expect great depth of focus. Under ordinary conditions, of light and distance, with fair judgment, and with lenses not too long in focus, these opposing qualities may be happily combined, so that lack of depth is hardly perceptible. Some apparent exceptions may be stated, for instance, a lens which produces images of general "softness," i. e., a lens in which the aberrations are not corrected to the utmost perfection. Such lenses, which lack snap and brilliancy, may show greater depth of focus than a firstclass lens. There is less difference between the "sharpest" focus and the image of objects forward and back of it, simply because the "sharpest" focus itself is not really sharp. Thus the statement that one lens has a greater depth of focus than others of the same aperture and focus, must be regarded as a rather doubtful compliment to the lens, for as stated above, depth of focus cannot be made subject to special correction. Another case may be mentioned in which one lens may really have an advantage over another one, in regard to depth of focus. In some constructions correction of astigmatism is obtained at a great sacrifice of simplicity by employing an unusual number of lenses separated by air spaces. There is a certain loss of light by reflection on a lens surface and it is easily intelligible that the fewer reflecting surface in a lens, the smaller the loss of light. In some constructions the number of the lens surfaces runs up as high as ten, while the Tessar contains only six. The consequence is that the lens with the greater number of reflecting surfaces requires a longer exposure than a lens of simple construction, although both may have the same relative aperture. Or to express It differently : the lens with the greater number of reflections requires an aperture of F:6.3 with a certain time of exposure, while the other lens will give a negative of equal density with its aperture stopped down to F :7.2 or F :7.5, which means a gain in depth of focus for the lens with the smaller number of reflecting surfaces. ♦Copyright, 1917, by the Chalmers Publishing Co. Cinematograph lenses are usually made with the smallest number of reflecting surfaces consistent with the requisite correction. They are also slightly faster than larger lenses of equal aperture because their small size makes the glass to be traversed by the light much thinner. Spherical Aberration. Owing to the fact that lenses are made with spherical curves, all single collective lenses have the defect of imaging an object through their' marginal zone at a shorter focus than through their central zone. Such a lens may give a sharp image with a small central diaphragm, and a sharp image as well if the center is covered with a round opaque stop so that only an annular zone around the margin comes into action. But both images will not lie in the same plane, nor will they be of the same size. Even if a lens is spherically corrected, so that the parallel rays penetrating the lens near the optical axis and those going through the lens near the margin come to exactly the same focus, there may be a slight remnant of spherical aberration in the zone between center and margin. Small remnants of this kind (so-called Zonal Errors) are found in almost all photographic lenses, especially of the cemented symmetrical type. The unsymmetrical combination upon which the Tessar construction is based, allows a better correction of the zonal errors than any other known construction. The greater the relative aperture (speed of the lens), the greater the task to correct the spherical aberration for all zones of the lens. Unsatisfactory spherical correction is indicated either by a general indistinctness of the image or by a fairly sharp image, which is entirely covered by halo (fog). Stopping down the aperture may improve the performance of a badly corrected objective. Coma. The spherical aberration of pencils of light going through the object in oblique direction is called coma. This manifests itself in the fact that although objects in the center of the field appear perfectly defined, objects outside the center show a one-sided indistinctness which increases towards the margin of the field, and in the image of a pointshaped object assumes the form of a tail like a comet, wherefrom this aberration takes its name. Stopping down reduces the amount of coma. Astigmatism. Astigmatism is that aberration which withstood longest the efforts of the opticians. A lens which is not corrected for astigmatism will not image sharply horizontal and vertical lines at the same time near the margin of the plate, although the center of the image may be perfect. This aberration is inherent in narrow pencils of light, so that stopping down the lens will not decrease the amount of antigmatism to the same degree that it decreases other uncorrected aberrations. In the absence of a test chart a very simple test for astigmatism may be made by focusing on the joints of a brick wall. No matter how much the lens may be racked in or out, both horizontal and vertical lines will never be sharply defined at the same time near the margin of the plate. Curvature of field. The ordinary lens images a flat object, not in a plane, but in a spheroidal surface, so that when the center of the image is focused sharp, the ground glass has to be brought nearer to the lens to obtain a sharp image of an object point near the margin of the plate. It is only in recent years that it is possible to correct astigmatism, together with the curvature of field in lenses of high speed. Lenses which are free from spherical aberration for a large aperture and produce a flat image free from astigmatism, are called "Anastigmats," the prefix "an" meaning without, hence, without astigmatism. Distortion is that fault of a lens which prevents the rendering of straight lines as such. The straight lines are reproduced as curves. All single lenses used with a diaphragm in front (landscape lenses) are subject to this defect in some degree. The distortion is called cushion shaped,' when the curves are concave, and barrel shaped, when the curves are convex toward the margin of the plate. Lenses which are free from distortion are called rectilinear. The performance of a lens which distorts cannot be improved by using smaller stops. Distortion has nothing to do with curvature of field. The image can be properly flat and the definition perfect, and yet straight lines may be distorted into curves. Chromatic aberration is due to the fact that in a lens, unless corrected from chromatic aberration, the visual rays which form the image seen on the ground glass do not form the images at the same position as the actinic or chemical rays, which affect the sensitive plate. Since the image is focused with rays for which the eye is most sensitive, the image formed by the rays for which the plate is most sensitive will fall outside of the visual focus (focal point), and therefore must be blurred on the plate. Of course all photographic lenses which claim to be of any value at all must, first of all, be corrected for chromatic aberration. An objective which has chromatic aberration is sometimes said to have chemical focus. (To be Continued)