Home Movies and Home Talkies (Jun 1933-May 1934)

HOME MOVIES & HOME TALKIES 95 1 to 8 ; /'4.5 means the proportion is 1 to 4.5 ; f'2 again means that the tUstance from the lens to the film is just twice the diameter of the lens opening. Yoa will thus see that the smaller the figure the larger the amount of light which will reach the film. It is not easy at first sight to see the relationship of exposure to lens opening or aperture, for if the opening is twice as great, not twice but four times the amomit of light is permitted to enter. You will realise this if you <lraw two circles, one twice the diameter of the other. You will see at once that the available area is obviously increased as the square of the number. The Iris Diaphragm The alteration of the opening of the lens is usually effected by a device known as an iris diaphragm, a clever piece of mechanism copied from the human eye. Jf you examine your camera lens carefully you will be able to see the movement of this iris <liaphragm as you move the little lever or ring which controls the "/ " numbers. Lenses are usually marked with their maximum aperture and then with a scale showing the various apertures to whicVi it can be ' ' stopped do-wn." Generally .speaking, the consecutive stops are marked with openings which give double the exposvire, for example, 4, 5.6, 8, 11, 16, etc. ; //8 allows twice the amoimt of light of//ll,//ll twice the amount of//16, and so on. If at any time you want to compare apertures, particularly when they are not whole numbers, all you have to do is to take the two figures and compare their " squares." For example, if you want to find how much faster /, 4 is than //8, all you have to do is to take the square of 4, which is 16, and the square of 8, which is 64, and you will see the proportion is 4 — 1. I have taken this example because it can be done quickly in the head, but, of course, you will require a piece of paper to work out how much faster /'1. 9 is than //3.5. Try it and you will see it is about 3J times as JFast. An //1. 5 lens may not soimd to you very much faster than //1. 9, but you work it out and you will find that it is over half as fast again ! Shutter Speeds The other way in wliich we can vary the amoimt of light reaching a film is by varying the amoimt of time the shutter is open. With a .still camera this can be varied over wide limits and thus we have shutters marked to give everything from one second to 1 /300th or so. In the cine camera, however, we have a difficult problem here, for our standard speed of taking is sixteen pictures a second (in the Pathe cameras the standard speed is fourteen a second), and, of course, we have to allow a certain amoimt of time for moving the film on. Actually, with a cine camera running at normal speed 1 '32nd of a second is occupied in moving the film along and another l/32nd for the exposure. A few very higii grade cameras are so perfectly made that it is possible to allow as long as l/27th of a second for exposure but this represents the slowest exposm-e at which a cine camera running normally can be operated. It is not convenient, save in special cameras, to vary this speed, and so you can reckon in cine work the shutter speed is constant at about ]/32nd of a second, and our only means of varying the light admitted and therefore controlling the exposure is by varying the lens opening with our diaphragm. What Opening ? Wliat lens o])ening, then, shall we use ? This depends upon one thing only, and that is the amount of light reaching the film from the subject. This amount of light is controlled by the intensity of the light source and have mentioned above, films vary in speed and we must know the material we are using before we can calculate our exposures correctly. There are several kinds of cine film available and a brief consideration of these is necessary before we go any further. Cine film first of all can be divided into two kinds, the "reversal" and the "negative-positive," usually abbreviated to " neg.-pos." With reversal film the actual strip of film you take in the camera is developed in such a way that it is turned into a positive ready for projection. In the neg.-pos. film the film in your camera is developed as a negative just as your spool of still pictures is developed as the negative, and from this a positive print is made. Each kind has advantages and disadvantages which I do not projDose to discuss here, but so far as speed is concerned they do not necessarily differ. Another classification of Don't miss shots like this during your holiday station-mistress in Wales, at Tan-y-BwIch Miss Betty Jones, the only the range of brightness of the subject. An ordinary street scene, for example, on a bright sunny day in August with no deep shadows anj'where will probably require on normal film an opening of //8. Another scene taken at the same time of day, but imder the trees in shadow, may require our largest opening of //3.5. Again, o\ir same street scene on a dull day will reflect much less light and will require perhaps //4.5. There are only three ways of finding out what is the correct exposure — to find out by the expensive method of trial and error and experience, to work out the exposure from a set of tables, or to use one of the many devices sold under the name of exposure meters which give a more or less accurate measurement of the exposure required. Film Speeds Then there is the question of the sensitivity of the film used. As I film is into " ortho " or, giving it its full spelling, " orthochromatic," and "pan" or "panchromatic." Orthochromatic film is only sensitive to certain colours of light and being completely insensitive to red it can be developed in a red light and loaded into chargers when they are not of the daylight loading variety. Amateurs who like to develop their own will naturally use the orthochromatic variety of film as they can see what they are doing in the dark room. Panchromatic film, on the other hand, is sensitive to all coloiu's and has to be developed and loaded in complete darkness. Actually, orthochromatic film is unduly sensitive to violet and blue and has only a little sensitivity in the green and yellow, or expressed in another way, it has not a faithful way of depicting the scenes. Panchromatic film being sensitive to all colours gives a much better rendering