As regards the efficiency of a shutter, the question is as to how it is to be judged. It is not difficult to understand, if we have two shutters, both of which give the same total duration of ex posure, that if one leaves the full aperture of the lens uncovered for double the time that the other does, the first one will allow a greater quantity of light to reach the plate, and will, there fore, be more efficient than the second. In other words, the efficiency of a shutter depends on the ratio of the time during which the plate, or part of the plate, receives the light coming through the full aperture, to that of opening and closing. Perfect efficiency, of course, would be when the time of opening and closing is nil, and the full aperture is used throughout. If this theoretic efficiency could be secured, it is manifest that it would be immaterial whether the shut ter were at the back, the front, or in the center of the lens. In comparing the efficiency of one shutter with another, the standard that is taken as unity is the theoretical shutter, but every part of the plate has to be considered, unless the shutter be placed at the diaphragm. What we have to determine, then, is the aperture uncovered during every small interval of time, to add these apertures together, and divide by the total duration of exposure, and compare it directly with the full aperture. Thus, suppose the total duration of the exposure was the second, that
At the of a second, of the area of the aperture was uncovered.
At the of a second, of the area of the aperture was uncovered.
At the of a second, -41 of the area of the aperture was uncovered.
At the -2-4 to the the whole aperture was uncovered.
At the of a second, 11- of the aperture was uncovered.
At the of a second, - of the aperture was uncovered.
At the of a second, of the aperture was uncovered.
The efficiency would be + i) 2 + 3, or 61 approximately, whilst the theoretical perfect efficiency would be io. The relative efficiency would be 6 — , or .64. If the areas uncovered for io another shutter of the same speed were respectively A, 4, A„ A, the efficiency would be only .27.
If a plate, therefore, with the first shutter was just enough exposed, with the second it would be greatly under-exposed.