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120 THE ADVANCE OF PHOTOGRAPHY
V
that is v \ ^xKr^2=~r (u+v)
1+1 1 /
U V f
This method is particularly useful, since the only measurements required are distance of screen from some fixed object, the length of this object, and the length of the image formed on the screen.
It is as well to bear in mind that no great advantage accrues from using a lens larger than is necessary for the particular sized camera, in fact it may be a distinct disadvantage, owing to loss of brightness in the picture due to reflections in the camera.
The circle of illumination of the lens should be known, because if it only just covers the plate when the camera is horizontal, when the camera is tilted it may be unable to do so. If the circle of illumination is known, then the amount of movement that can be given to the lens is also known, and likewise the largest size of plate which can be used with the lens.
The rapidity of the lens depends upon the amount of light which can pass through it to the sensitive plate ; this must of course depend upon the size of the stop used and the distance of the stop from the plate. The ratio,
Diameter of largest stop giving the required degree of sharpness Focal length of the lens,
is known as the intensity ratio, and this is a gauge of the speed at which the lens works, so that the larger the denominator of this ratio, the slower the lens. Thus, suppose the diameter of stop is J" and the focal length of the lens 8", then J/8=^ — i.e. this system has what is known as an intensity of //16. The / values of