Movie Makers (Jan-Dec 1938)

290 need is the familiar portrait attachment, or a supplementary lens, which is offered by nearly every camera manufacturer who has fixed focus objectives for sale. Portrait attachments for use at distances of three and four feet from the camera to the subject can be obtained readily. They are mounted, like filters, in a cell that can be slipped over the lens mount, and, in the case of some of the older 16mm. cameras, a holder for these lenses was built into the mechanism housing. The chief precaution in using a supplementary lens or portrait attachment is to be sure that it is removed before making scenes other than closeups. If a one inch focusing lens is set for a distance of twenty five feet and is used at diaphragm stops from f/5.6 to //16, it will function as a fixed focus lens. This is particularly advantageous in filming parades, football games, rodeos and other events in which the distance from camera to subject is changing continuously. Next in the list of the four methods of obtaining correct focus comes the use of a tape measure. This method may seem cumbersome, but, nevertheless, theatrical cameramen always "tape" each scene. It is easy enough for the amateur movie maker to use a tape measure when the subject is relatively close to the camera, and it is within these shorter distances that it is most necessary to have accurate focus. At distances over ten feet, the depth of field will cover any small error in guessing at the focus setting. A light tape does not take much room in the camera case, but using it calls for the help of an assistant, unless the camera is on a tripod and one has devised a simple hook which will enable him to hitch one end of the tape beside the lens. A small piece of adhesive tape might be substituted for the hook. Distances for ordinary closeups may be measured from the diaphragm of the lens to the subject. This is accurate for all work except ultra closeups, where it may be necessary to measure from the plane of the film. Filming ultra closeups with extension tubes is a specialized field, and the camera manufacturer should be consulted when the tubes are purchased so that correct information may be obtained for the particular outfit in use. When selecting the part of the subject on which to focus, it is well to remember that the depth of field beyond the point of focus is greater than it is between that point and the camera. Thus a subject covering a distance of three feet would be in better focus throughout its depth if the focus were determined on a point at about one third of the distance from the front to the back of the object. The third method of focusing is by means of the range finder, or distance meter, of which several types are on the market. Their use is indicated especially when it would be difficult to measure or to pace off the distance from camera to subject. For example, in making shots over a pond or brook, it would be either impossible or most inconvenient to measure the distance by tape. Most meters work on the principle of triangulation. and, when one sights through the device, he sees two images. As the adjusting wheel is turned, one of the images moves to the side and eventually is superimposed on the first image. When the superimposition is accurately completed, the distance may be read on a dial. Other types split the image into two parts, which must be brought into line to find the correct reading, and still another involves lenses like those used in a camera. When the object has been brought into sharp focus on a tiny ground glass, the distance is read from a revolving scale. In at least one 16mm. camera, a range finder is built into the mechanism so that, when the distance has been found by triangulation. the lens is automatically set for the correct point. Still photographers are generally familiar with this idea, as it has been used widely on the miniature type of cameras. Range finders are made to cover distances less than one hundred feet, since most lenses reach the infinity point at about that distance. A distance measuring device of sufficient scope and accuracy for use with the longer focus telephoto lenses would be somewhat larger than those available at present, because it would require a longer base for the operation of the triangulation principle with the requisite accuracy. The fourth method of focusing involves the use of all visual devices which are built into the camera, or attached to it, and which may be grouped in one general class. Such visual focusers usually incorporate: (a) a ground glass screen, on which the image is focused by means of the taking lens; (b) a mirror, or prism, by means of which this image is reflected into a more convenient position for viewing; (c) a magnifier for enlarging this image so that its details can be seen more clearly by the eye. When no mirror is used to deflect the path of the light rays in such a device, it is called a direct visual focuser. When a mirror or prism is used, it is called a reflex visual focusing device. If the camera gate were cut out directly back of the taking aperture, and if we put a piece of matte or ground film in the gate, by looking through the back of the film, we could see the image clearly, as focused by the lens in the taking position. This would be the simplest form of direct visual focuser. If we could view the image through a magnifier, our perception would be improved. Some 35mm. cameras do have a direct viewing tube that runs from back of the gate, out through the back of the camera. By looking at an eyepiece at the outer end of this tube, one may see through the back of the film in the gate and may focus the image in this way. This device is known as focus on film. Naturally, the camera shutter must be open while this is done, and, of course, negative or positive film, which has no opaque coating, must be used. Certain 16mm. cameras, mostly of European make, are equipped with this device, either in direct or reflex form. It is useful only with unbacked film, or with matte film, placed in the gate temporarily. Such a device, of course, gives an actual view of the image formed by the taking lens, in the taking position, and is useful for checking the performance of lenses, or for any kind of centering and closeup work. A modern focusing device, which is simply an extension of this direct focusing principle, operates in conjunction with a well known magazine camera. In place of the magazine, there is inserted a frame which covers a tube. This tube has a ground glass at one end and an eyepiece at the other. When in place, the ground glass surface occupies the same position in the aperture as does the film. Hence, with the shutter open, one may see and focus the image on the ground glass by looking through the eyepiece at the other end of the tube. A noteworthy application of the principle of visual focusing on a ground glass, placed in the aperture in the same position as that occupied by the film, is used in an advanced type of 16mm. sound on film camera. In this built in device, turning a knob moves the film aside and replaces it with the focusing screen. Focusing and centering then are done through the taking lens in the taking position, after which the film automatically is returned to position, ready for shooting. If a ground glass focusing screen is placed in the camera somewhere near the taking position, and in exactly the same plane as that occupied by the film in the gate, one may focus the lens accurately on this ground glass and be assured that the focus will be accurate on the film. This principle is used in some cameras which have turret mounts. When the taking lens is revolved into another position on the turret, it does not move toward or away from the film plane. Hence, a ground glass focusing screen may be placed conveniently so that the lens may be kept in line with it, when the turret is moved to bring it into some position other than the taking position. This movement of the lens, being sidewise only, will permit accurate focusing, but not accurate centering, because here the problem of parallax is involved. The critical focusing device on one camera is based on this principle. The taking lens is revolved in the turret until it is brought into line with a small, circular, ground glass screen. The image formed on this screen is brought to the eye through a small reflecting prism and a magnifier. This particular device covers only a portion of the center of the picture field, and, while this portion is greatly magnified, giving a practical means for critical focusing, it cannot be used for placing or composing the elements [Continued on page 304]