19. Convertible Three-Focus Lens

Convertible Three-Focus Lens. With the convertible three-focus lens can be obtained similar results to the Telephoto, as by using the rear combination alone the image is doubled in size. Removing the rear combination entirely and substituting the front produces a still larger image. "With even the ordinary rectilinear lens, a larger image can be obtained by removing the rear combination and using the front lens alone.

20. The use of these various combinations is very convenient when it is necessary to photograph at long distances. Often an object photographed with the regular lens will be quite indistinct, where if the single combination was used it would be much larger and more clearly visible. It must be remembered, however, that by changing the combination of the lenses you also change the working speed. Collectively the lens may be very rapid, yet, when used singly, four times the exposure will be necessary. It is also imperative in using this class of lens to have an extra long bellows, as a short one would not allow for proper focusing. When not using the full length of bellows hook the ring, which is attached to the top of bellows, to the hook over the front board. This will obviate the sagging of the bellows.

21. Shutters

Shutters. Shutters of the "Bausch & Lomb," or "Wollensak" time and instantaneous registering variety, can be set without opening the shutter, and are most convenient. They are not expensive, nor are they extremely rapid, ,'but as architectural photography does not require extreme rapidity of exposure, they answer all purposes. These shutters are fitted with Iris diaphragms. By means of the Iris diaphragm you can graduate the size of the aperture without interfering with the shutter, a feature which makes this class of shutter most desirable. If a more rapid shutter is desired, the Volute or Sector may be employed. These shutters in the smaller sizes, up to 5x7, are regulated so as to give an exposure from 1-150 to 3 seconds.

22. Diaphragms Or Stops

Diaphragms Or Stops. The uses of diaphragms, commonly called stops, in a lens are numerous. By the use of small stops the picture is made sharper, more definition is obtained and the depth of field covered by the lens is increased. In architectural photography it is necessary to use a small stop to meet severe requirements; for instance, photographing a tall building, which necessitates the extreme use of the swing-back. This makes a division of focus between the upper and lower portion of the plate necessary, and in order to produce sharpness throughout the plate a very small stop must be used. While the small stop admits less light upon the sensitized plate, and necessarily prolongs the exposure, it gives greater latitude, greatly reducing the chances of failure. As the architectural photographer seldom finds it necessary to make short exposures the stopping down is not objectionable. By the use of a small stop the contrast in the negative is increased, while the shades and shadows become deeper and sharper. The highlights are thus made stronger. If, on the contrary, a soft negative, free from contrast is desired, this effect can be produced by the use of a large stop.