Motion Picture News (Mar-Apr 1923)

1478 Motion Pi c t u r e News Fundamentals of Light for Cinematography Continuation of Article on Relation of Principles of Physics to Photography Extracts from " Motion Picture Photography " by Carl L. Gregory, publishea oy Neic York Institute of Photography Chapter III (Continued) UP to this point we have only considered light as it emanates from a point, but now we are ready to consider any object which may be reproduced by a lens as an image. In photography, practically all Pig i4_ Production o) an image by a lens images that we have to consider are delineated or formed in one plane; that is, either upon the flat surface of a photographic plate or upon a firm stretched flat or upon a piece of photographic paper, as in a photograph, or upon a screen in a moving picture theatre, so that no matter by what means we produce a photographic image it is practically always done upon a flat surface. Let us, for the purpose of our analysis, consider any object or anv image as being composed of a collection of a vast number of small points of different degrees of illumination placed beside each other, forming an infinitely fine mosaic which delineates the object or image which we have under consideration. To make this point clearer, inspect very closely with the naked eye, or better still, with a small magnifying glass, any half-tone cut in this or any other book or paper and you will see that the entire picture is formed by small dots of varying sizes which make up the picture. In the same manner we may consider any object or image as consisting of an infinite number of small points not necessarily arranged in mechanical order as in a half-tone cut. This mechanical sequence in a half-tone is merely a method of surmounting certain mechanical difficulties in photo-mechanical reproduction, the size of the dot representing the intensity of illumination of that particular portion of the picture which it represents. There are many other processes of photogravure too complicated for. ordinary book production in which the dots are arranged in irregular order or in which the light intensity is registered by other means, such as the Mosstype, the Albertype and various photogelatine and lithographic processes. We have already seen that all objects reflect a certain percentage of light. If by means of a lens we can focus the luminous points which delineate an object upon a flat surface, we must necessarily obtain an image of that object upon the focal plane, as in Fig. 14. This image is always reversed and inverted; that is, like a mirror reflection turned upside down. By again referring to Fig. 14 we see the reason for this. All of the light rays emanating from A on the tree which -trike the lens are condensed and brought to a focus at the point a in the image. Likewise, all of the rays which strike the lens from the point B are focused at the point b in the image: in a like manner all of the other points on the surface of the tree are delineated on the screen without rendering the diagram too complicated by trying to reproduce the path of the light rays from all of the other points on the tree. If we move the screen a small distance in either direction from the focal plane the image becomes blurred and indistinct, since our points of illumination then become overlapping circles of confusion, as in Fig. 15. The image ab in Fig. 14 is termed a real image, because it may be focused upon a screen and to distinguish it from certain other images which we will consider later, which can be seen but which cannot be focused upon a screen and which are termed virtual images. This image may be again focused by another lens which again inverts the imase. as in Fie. 16. Rig. 15 — Indistinct image caused by overlapping circles of confusion In Fig. 17 we have a diagram in the ordinary telescope in which the real image has been twice enlarged, in order that the eye may see the enlarged image as an erect object. As it is of no consequence that the image be inverted in an astronomical telescope, it is provided with only two sets of lenses and the image is enlarged but once, the large lens, or objective, being made as large as possible in order to collect all of the possible light from dim and distant stars. The image formed by this large objective with great light collecting power being then examined by a magnifying eye-piece selected by the astronomer as being most suitable for whatever investigation he is conducting: large astronomical telescopes being provided with a number of eye-pieces of various degrees of magnification. When photographs are taken of heavenly bodies the eye-pieces are removed and the photographic plate inserted in the tube of the telescope at the proper focal distance. In our experiments with the prism we learned that the glass of the prism had not only the power of refracting or bending the light, but also of dispersing or separating it Fig. 18 — Light dispersion caused by an uncorrected lens into its component colors, and in our previous i xperiments with a single lens we will have noticed, if we have observed closely, that the images which we produced were fringed with prismatic color. In diagram 18 we see the reason for this, the blue and violet rays being refracted to a greater extent than those of the other end of the spectrum. Very happily for photographic purposes, the light reflecting power and the dispersive power of different kinds of glass are very different and not interdependent so that we are able to produce by cementing together, as in Fig. 19, or sometimes only mounting together in a metallic mount, lenses from certain combinations of different kinds of glass in which one kind counteracts the dispersive power of another kind and thus the different colors are brought to a focus at the same point. It would be very inconvenient to make a mathematical calculation and a very fine readjustment of a ground glass from the visual focus to the actinic focus of a lens every time we wished to take a photograph. This correction for visual and actinic focus is thus very important and is one of the principal reasons an ordinary magnifying lens is not suitable for making photographs. It is an unfortunate fact that there are on the market today some makes of cinematographic lenses which are not fully corrected Fig. 16 — Double inversion by means of tico lenses. Fig. 17 — Diagram showing the path of the light rays in an ordinary telescope l i<j 19 — Correction of dispersion by lens elements of different kinds of glass for visual and actinit focus. The writer was at one time compelled through force of necessity, to use such a lens, and it was only after making many tests to obtain a focusing scale or by focusing upon an object at a certain ratio of distance nearer the lens, that he was able to produce pictures of satisfactory sharpness with it. As it is never necessary to change the focal distance from infinity in astronomical photography, no attempt is made to correct telescopic objectives since, when actinic focus is once obtained it is never necessary to change it. The lens is the agent by which the light is directed to the right spot in forming the (Continued on pane 1484)