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AMATEUR MOVIE MAKING 97
termines the exposure. Thus the amount of light which falls upon the subject has a very definite bearing upon the lens aperture necessary to photograph this object at normal speed.
Intensity of Light. — We know that the intensity of any light varies inversely as the square of its distance. Thus if we have our light at two feet distance and move it to six feet distance, or three times as far away, only oneninth as much light will fall upon the subject. This is the second factor in judging the amount of usable light reflected from any surface. We must then consider intensity and space or distance. Naturally, as we have seen before, the photographic exposure is determined, among other factors, by the duration of the exposure of the film to light. Here we have the time-space-intensity equation. These are definite factors, the time and intensity being fixed and the space having a definite bearing upon the calculation. Thus if with a given light we can make successful exposure at f 8 and we move that light twice as far away we know that we must use four times as much light, so we use stop f 4.
When the light is moved twice as far away, use onehalf the f value, with it three times as far away use onethird and so forth. Thus moving the light three times as far away would lead to the use of f 3 when f 9 had been the proper light before moving the light.
The f number varies inversely as the distance of the light from the subject.
We can now see that with a low initial light intensity we should use a large lens aperture, and with a high intensity light we should use a lens of small aperture. The question now arises as to which combination is better. If we have a light of ten thousand initial candlepower intensity, which enables us to make a satisfactory negative at f 3.5, it is evident that a light of twenty-thousand initial candlepower would enable us to use an aperture whose area is one-half that of the f 3.5. This would be roughly f 5. Conversely, with a lens of approximately f 2.5 speed, we could secure the same results using a light of approximately five thousand candlepower.