_f_/3·5 gives an aperture 3/5-inch in diameter (nearly) 4 " " 1/2-inch " 5·6 " " 1/3-inch " " 8 " " 1/4-inch " 11 " " 1/5-inch " " 16 " " 1/8-inch "
Although the differences between these successive apertures are very slight, they exercise a very appreciable effect upon the volume of light passing through the lens, and accordingly the period of the exposure. Thus although stop _f_/5·6 only decreases the size of the aperture by 1/6 of an inch over _f_/4, yet the effect of this reduction is to necessitate twice as long an exposure as is suited for the latter stop. Similarly _f_/8 demands twice the exposure of that required for _f_/5·6, and so on, the exposure being doubled with every diminution of the stop up to the limits of the diaphragm. Yet in practice this increase of exposure between two stops is impossible, because the handle must be turned at a definite speed. It is obvious, therefore, that compensation must come from another quarter. Instead of increasing the duration of the exposure we must have a greater intensity of light for _f_/5·6 than for _f_/4.
At first sight the beginner might be disposed to think that the selection of the most favourable aperture is a matter demanding extremely fine judgment and skill, especially when there are other factors which may upset calculations. Many other advantages arise from using as small an aperture as possible, such as increased sharpness of the picture, especially at the edges. Mr. Williamson the designer of the camera has realised this, and as a result of his unique experience, he has set down some very useful rules to guide the beginner, as to which stop should be used for varying conditions of light and subject. In elaborating this advice Mr. Williamson rightly commences from the zero point as it were, taking moving-pictures of a football match on a dull winter afternoon, when, owing to the feeble light, the capacity of the lens and the sensitiveness of the film are strained to the utmost. From this point he has graduated the diaphragm and its use as follows:--
-------------------------------------------------- Stop. | Subject and Conditions. -------------------------------------------------- F/3·5 | On a dull winter's day; well-lighted | interior: or on a subject at any time | of the year where there are heavy | shadows such as under trees. -------------------------------------------------- F/4 | On a bright day in winter: on dull | days in spring and autumn. -------------------------------------------------- F/5·6 | Outdoor exposures during September, | October, March, and April. Dull | summer weather. -------------------------------------------------- F/8 | Street scenes in bright summer | weather. -------------------------------------------------- F/11 | Open fields in bright sunshine. -------------------------------------------------- F/16 | Bright sea and sky subjects. --------------------------------------------------