| f/ | 3·5 | gives | an | aperture | ⅗-inch | in diameter | (nearly) |
| 4 | " | " | ½-inch | " | |||
| 5·6 | " | " | ⅓-inch | " | " | ||
| 8 | " | " | ¼-inch | " | |||
| 11 | " | " | ⅕-inch | " | " | ||
| 16 | " | " | ⅛-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 ⅙ 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. |
It must be understood that the foregoing are not set down as hard and fast guiding rules, but they may be safely taken as some indication of what should be done under such varying conditions. They may be said to apply generally to the temperate zones where the conditions are almost identical, irrespective of geographical situation. If the beginner follows them at first he will not make very serious mistakes. But, as has been said, experience alone can finally determine the factor of lens aperture.
The size of the aperture has another far-reaching effect. This is in regard to focussing. With the 2-inch Zeiss-Tessar lens of the Williamson camera when the largest aperture is used, nothing important in the picture should be within a distance of 20 feet. If it is, it will not be in focus. As the diaphragm is closed this distance decreases proportionately until the infinity, INF, mark is reached. At this point practically everything is in focus. The distance when other objects are in focus at the respective stops is as follows.
| f/3·5 | focus | distance | 20 | feet |
| f/4 | " | " | 20 | " |
| f/5·6 | " | " | 15 | " |
| f/8 | " | " | 12 | " |
| f/11 | " | " | 10 | " |
| f/16 | " | " | 5 | " |
| INF | " | " | everything. | |
At first sight the fact that the camera is operated by the turning of a handle makes it seem to be absurdly simple. One or two experiments however, will prove that it is far from being as easy as it looks. The salient point is to turn the handle steadily and evenly so as to complete two revolutions per second. The first pictures will be found to be very unsatisfactory, having an eccentric jerky effect instead of a smooth easy animation. An even pressure must be maintained throughout the complete rotation, and, before the beginner attempts to take any pictures and thereby waste expensive film, he would do well to practise handle-turning until he has become proficient. If the turning movement is timed with a watch, and "one" corresponding to a second is counted for each double turn, a perfectly steady turning movement will soon be attained. Some cameras are fitted with an indicator which records the number of feet of film consumed. But no anxiety need be felt if this convenience is absent. The operator need only count one, two, three, and so on, while turning the handle, each number representing a double turn. In this way, as 16 pictures, equivalent to one foot of film, are made with every double turn and every one count, the number reached at the end of the task will show how many feet of ribbon have been used, and if this is deducted from the amount originally held by the loaded box it is easy to tell the length of film unused. When the upper box has been exhausted and the lower box filled, the latter is withdrawn and packed away to be opened in the dark room only. The empty upper box is taken out and slipped into the lower position to act as a receiver from the next loaded film-box.
In photographing, the operator must keep his eyes riveted upon the view finder, to make sure that the subject he desires is in the field of the lens. The movements can be followed easily, and there should be no difficulty in keeping the most important part of the subject in the centre of the picture.