FALLACIES REGARDING EXPOSURES.
First among these is (a) the impression that different lenses vary in rapidity, although similar stops are used. The amount of light reaching the plate depends upon the size of the hole by which it is ad mitted and the distance over which it has to travel ; therefore, when the diameter of the stop bears the same proportion to the focal length, the intensity of the light will, for all practical purposes, be the same. Theoretically there are certain other con siderations, such as the number of reflect ing surfaces, but these for the present may be ignored. (b) It is frequently supposed that if a whole-plate lens be used to form a half-plate picture, less expos u•e will be required. A moment's con sideration of the facts proving (a) will show this also to be wrong, as here again the amount of light received by the plate is represented by the proportion the stop bears to the focal extension, it being an unimportant point whether the whole of the picture formed by the lens is received on the plate or only a portion of it. At
the same time, where the field given by the lens covers a much larger area than the plate can accommodate, the remainder is distributed over the interior of the camera, and may, if reflected back, influence ex posure by the introduction of fog. (c) Again, it is often imagined that the posi tion of the camera in the sun or shade directly affects the exposure. The light used to form the image on the photographic plate is that which is reflected from the ob ject being photographed ; but the amount of light reaching the camera may differ considerably from the amount reaching the object and reflected to the camera, and therefore cannot he taken as a guide for exposure.