683. Angle Of View

Angle Of View. The extreme width of angle which may be obtained with a pin-hole renders its aid very valuable when working in confined situations. With a very carefully made pin-hole it is possible to produce a much wider angle of view than could be secured with the majority of the wide-angle lenses on the market. It is, however, very important to have the edge of the pin-hole perfectly clean and smooth, and if the angle is extremely wide, the cleaner and thinner the edge, the better and more evenly illuminated will be the result. For pictorial work a wide angle is not recommended; in fact a very narrow angle will give far better results - one of about 30 degrees. The wide angle will prove of value for record work and producing photographs, in very restricted areas, required for commercial purposes.

684. The nearer the pin-hole is to the plate the wider is the angle of view, and the shorter is the exposure required. With the ordinary rectilinear lens, when taking a distant object, it is necessary to work practically at one distance only from the plate, whereas with one and the same pin-hole it is possible to work at any distance from the plate. For example, a number 10 hole will cover a 5 x 7 plate, at one and a half inches distance. This gives an angle of view of about 128 degrees, but at whatever distance the pin-hole is from the plate the image is equally in focus. Of course the further the pin-hole is from the ground-glass the less will be the angle of view. These are facts of great importance, as the one pin-hole supplies the place of a whole battery of lenses.

685. Exposure

Exposure. There are two principal methods employed for ascertaining the required exposure for pin-holes of various sizes, and for the various distances between the pin-hole and ground-glass. One method is to give the pinhole number 8 a factor of 7, say f/7, if the camera extension is one inch. If the camera extension is doubled (two inches) this factor is doubled - f/14. If the extension is three inches the factor will be f/21. Whatever the number of inches of extension the factor is found by multiplying the number of inches by f/7. If using the Watkin 's Exposure Meter, whatever time it gives in seconds, use minutes. As an example, say we are working at f/14, and the meter gives six seconds, then expose for six minutes. If you do not employ an exposure meter to judge your exposures with a lens, take the number of seconds, or fraction of seconds, you would give with the lens at a particular diaphragm value and use this in minutes for your pin-hole exposures. Remember, however, that it is necessary to take into consideration the f values at all times.

686. A second method is worked out on a different basis and involves a little mathematical calculation, but when the principle is once understood it will be found to be extremely practical and very accurate. The method is simplified by certain ascertained factors:

For a No. 8 hole the factor is 32,

For a No. 10 hole the factor is 55,

For a No. 12 hole the factor is 90.

Take the exposure you would give if you were using your lens at f/8, multiply it by the factor for the size needle-hole you are using, and again multiply the result by the square of the number of inches between the hole and the plate. For example, with a number 8 hole, and the distance 6 inches, and a lens exposure at f/8, of say 1/2 second, multiply 32 (the factor for pin-hole number 8) by 1/2 (the exposure for f/8), and the result is 16; multiply this (16) by the square of 6 (the distance between pin-hole and plate) - the square of 6 is 6 x 6, or 36 - and the result is 576, which is the number of seconds exposure required. To reduce this to minutes, divide 576 by 60 (the number of seconds in a minute), and the exposure will be a very little over 9 1/2 minutes. (9 minutes, 36 seconds.)