AN EXPOSURE RECOMPENSE In photographing near objects the lens rests further from the plate or film than it does when focused on a distant object. For example, should a lens of 16 cm. focus be 2 cm. in diam eter its convergence will be f/8 when focused on a distant object; but in drawing out the lens in order to focus on a near object, should it rest 2 cm. further from the plate than when focused for a distant object the actual focal distance is now 18 cm. and not 16 and its convergence is f/9 instead of f/8 as before. And should it be drawn out until it rested 16 cm. further from the plate than it does when at the long distance point it would be 32 cm. from the plate and its convergence would be only f/ 16 instead of f / 8. Since the latter is a convergence of 64 cone units and the former of only 16 it is seen that solely on account of the position of the lens the exposure should be increased four times, should the lens rest in the last position mentioned. In practice it is best to ascertain the exposure with any desired diaphragm by the usual method and as the last step observe the position of the lens and the increase of ex posure made necessary by it. The following method enables this recompense to be made very easily.
All film cameras have a focusing scale on which is seen a 100 foot or a 30 meter mark which in dicates the position of the lens when focused on distant objects. On cameras having no scale this mark should be made. This may be done by focusing with the open lens on any distant object and then making a mark on the solid part of the camera where the face of the sliding part which carries the lens rests. Now multiply the focal length of the lens successively by 0.19, 0.41, 0.67 and by 1 and at these measured dis tances out from the long distance mark make other marks on the camera bed and number them respectively 1.5, 2, 3 and 4.
Then on photographing any near object note the mark nearest which the lens rests and mul tiply the exposure as found under any conditions by the factor at that mark. For example should it be found under certain conditions of subject, light and diaphragm that an exposure of 2 sec onds was required and it is seen that the lens rests near the point marked 2, then the ex posure should be twice 2 or 4 seconds. Should the lens rest near the outer mark having the factor 4 then this exposure should be 4 times 2, or 8 seconds.