American cinematographer (Jan-Dec 1941)

Get Acquainted With Your Lens By CLAUDE CADARETTE Depth of field increases as lens is stopped down. Above, f : 1 2 .5 ; below f:2. THE lens of a camera is its eye. In the same manner that your eyes form an image on the retina of the eye-ball, the lens has the function of rendering an image on the film surface which is true in every respect to the original subject that is being photographed. Lenses are made in various combinations, starting from the singleelement lens, such as an ordinary reading glass, to very complex combinations that are developed for extreme sharpness, all corrections, and highest efficiency in light transmission. The single-element lens will form an image, but it does not have the ability to correct certain defects that occur. As we view the image formed by a simple lens, we notice that the image appears fairly good, but on closer examination we find that the sharpness is not all that is to be desired, and the edges of the image are distorted. These defects would be very obvious when enlarged on a screen by projection. As a result, the simple lens fails to render a true image of the subject and cannot be used in photography where absolute correction is needed. Therefore, we must use another type of lens that will overcome these defects by giving us a picture of uniform sharpness and a truthful reproduction of our subject. In speaking of lenses, let us find out what happens to a ray of white light as it passes through glass. Newton discovered that a white ray of sunlight is broken up into many different colored rays of light as it passes through a prism. This separation of rays is known as a spectrum. He noted that the blue rays of light which are at one end of the spectrum did not bend as much as the red rays which form the opposite end of the spectrum. The green and yellow rays were bent more than the blue but less than the red, and fell in their respective positions in the center. A simple lens is in fact a glorified prism or really two prisms placed base to base. The lens, however, concentrates all of these rays to a point that forms an image. If the blue rays don't bend at the same degree that the yellow or red rays are bent, naturally they will not all meet at a given point and as a consequence, the image loses some of its sharpness and the image appears fuzzy or out of focus. This defect is known as chromatic aberration. It was later discovered that certain types of glass had different powers of converging rays and that a combination of lenses made from flint and crown glass would make each colored ray of light converge at the same point, correcting the fuzzy effect caused by a simple lens. This correction, although very advantageous was not sufficient to give an accurate reproduction of a subject and more research was done to improve lenses to correct for distortion and add greater speed in light transmission. In motion picture photography, it is necessary to have lenses having all corrective fundaments as the image must be true when subjected to extreme magnifications on the screen. Speed is essential in motion picture camera lenses due to the fact that the shutter speed is constant and we are unable to make longtime exposures under poor light conditions. Science has given us the fast lenses but a means of controlling the amount of light entering the lens was necessary so that we could have a consistent exposure on the film. Founder. L. A. 8mm. Club Just as the iris of our eye opens in a darkened room to admit as much light as possible, or closes to a pin point in bright light, it was necessary to construct a flexible aperture in the lens assembly to give this same control to the camera eye. This light-valve in a lens is known as a diaphragm. The size of the openings of the diaphragm are calibrated so that each stop on the scale admits twice as much light as thr: previous stop. When a lens is focused on an object beyond one hundred feet from the camera, the distance from the diaphragm to the film surface or image formed, is known as the focal length of the lens. The focal length of a lens determines the area width and height that can be photographed. This angle of acceptance in a lens varies with the focal length, the angle decreasing as the focal length becomes greater. As an example, if we used a lens having a focal length of 1 inch, and can cover an area that is 30 feet wide, by using a lens with a focal length of 3 inches, we reduce the angle of acceptance to such a degree that the area covered is only 10 feet wide from the same camera position. The film records only rj of the original area and when projected on a screen, we get a telescopic result. For that reason, telephoto lenses are of a longer focal length but do not cover as much of the original area as the 1-inch lenses. The 3-inch lens will produce as large an image on the screen when photographed 18 feet from the subject as the 1-inch lens will produce at 6 feet from the subject. One can readily see the advantage this gives when filming difficult subjects. If you desire a closeup of some subject but are unable to approach it, the desired results can be had by the use of a telephoto lens. The actual size of the diaphragm aperture can vary, but the value is always in direct relation to the focal length of the lens. In other words, the opening of f:8 on a telephoto lens is physically larger than the f:8 opening on a ^-inch lens, but each opening allows the same amount of light to reach the film. T i compute the value of an opening, you divide the focal length of the lens by the diameter of the diaphragm opening and the result is its f-value. (Continued on Page 447) 432 September, I'll 1 A MKRICAN ClNEMATOGRAPHER