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MOTION PICTURE PHOTOGRAPHY
the lens but separated a short distance from one another, as in Figure No. 12 we will find if we have a screen for the rays to fall upon, that the two points will be reproduced side by side in exact miniature on the screen, but that the point of illumination which is above the original point of illumination is reproduced below the point of the original point of focus of the first point. If we now move this screen closer to or farther away from the lens, we find that the point of light enlarges in a circle of illumination. This is termed the circle of confusion. By moving the screen back and forth we also find that there is only one position in which the points of illumination are perfectly reproduced. If, however, we now move one of these points of illumination to a much greater distance than the other, we find that while one is sharp and distinct the other forms a small circle of confusion and that when we move the screen so that the more distant one is in focus, that the other becomes a circle of confusion, or out of focus, as it is termed. If, however, we move the two points closer to one another, but still at different distances from the lens, we find that we can bring them both to a focus on the screen or rather, so nearly to a focus that the eye is not able to distinguish the difference in sharpness between the two. This difference of distance between the two points of illumination is called the depth of focus.
Let us now take the points of illumination, as in Fig. 13, with one of the points focused sharply. If now we interpose a piece of black cardboard, in which a small round hole has been cut, close to the lens so that this hole is near the center of the lens, we find that the brightness of the images is much decreased but that the image of the point which was out of focus is now much sharper. Let us refer again to our Fig, 13. Our images are not nearly so brilliant because much of the light which formerly came throug'h the lens has been cut off by the piece of black cardboard ; but as the cardboard has narrowed down the angle which the light ray takes from the lens to the focal plane, we have narrowed down, or made smaller, our circle of confusion.
Up to this point we have only considered light as it emanates from a point, but now we are ready to consider any object which may be reproduced by a lens as an image. In photography, practically all images that we have to consider are delineated or formed in one plane; that is, either upon the flat surface of a
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