Types of Lenses

Modern shutters are either of the roller blind or improved sec tor kind. Roller blind shutters are of two separate types, one in which the blind as a whole opens or closes the lens, difference in speed being obtained by tightening or relaxing the spring of the actuating mechanism; the other in which a slit in the blind passes behind the lens, the speed of exposure being governed partly by the tension and partly by the width of the slit. The ordinary roller blind shutters, of which the Thornton-Pickard (fig. 34) is the standard model, are, within their limits, very effective and dependable. Speeds from about to about sec. can be given by adjusting the spring, and a separate attachment to the bulb enables time exposures up to 3 sec. to be made automatically. The theory of such shutters is explained by Abney in his Instan taneous Photography (1895) and by W. B. Coventry in The Technics of the Hand Camera 0900. Of the focal-plane shutter, as the slit blind variety is called, there are numerous patterns geared and with adjustable slits to work at speeds from to over sec. The theory is explained by Coventry (see above) and by C. Fabre in Traite encyclopedique de photographie, Vol. 1, Suppl.

C, 1902. This shutter works immediately in front of the plate and is normally built into the camera. The Goerz-Anschiitz pat tern is very typical of the entire group of focal-plane shutters.

The central lens shutter is now generally made with only two or three leaves or blades so constructed that practically the opening of all parts of the lens for exposure is simultaneous. Such shutters are made entirely of metal, and in the best patterns the mechanism is of the same precision as that of a watch. The exposure is effected by means of springs supplemented some times by an air-brake. A shutter known as the "Multi-speed" has been made to work between the lenses of a combination at speeds up to an alleged 0 sec., but in the best ordinary models a sec. is the highest speed attainable.

Remarkable advances have been made between 1929 and 1935 both as to the precision and effectiveness of shutters and, as well, to new adaptations of them. The trend is towards shutters that are fully automatic in operation and towards combination of functions with other camera devices. For example, a device controlled by photo-electric cell is now used for operating the diaphragm of photographic and cinematographic cameras accord ' ing to the intensity of light ; another device has exposure and iris adjustments coupled with light meter as an in-between-lens shutter; again, a Catadioptric Unit, whereby a controlled amount of diffusion is introduced, is offered. Cameras with

flash bulb circuit connections and synchronization are in creasingly popular. See also Photographic Shutters: Meth ods of Construction and Measuring their Speeds, by A. S. New man.

Exposure Meters.

The two most common types of exposure meters are actinometric and visual observation, the latter commonly known as "extinction" meters. In the former sensitized paper is exposed to the light fall ing on the object, and the expo sure is based on the time which the paper takes to darken to a certain tint. A narrow strip of specially prepared silver bromide paper which darkens rapidly on exposure to light is usually em ployed, a fresh section of it being exposed in juxtaposition with a fixed standard tint. The number of seconds, or minutes, counted before the standard tint is reached is called the actinometer time. This is correlated with the known speed of the plate and the stop in use, and the result read off on a dial. The leading British patterns of exposure meters are the Watkins and Wynne, both available in watch form. In the Watkins method plate-speeds are distinguished by numbers, in the Wynne meter by an F num ber, both designations being roughly comparable with the more precise H and D measurements. In "extinction" meters the visual intensity of the light reflected from the shadows is measured by direct observation. In many patterns, of which the "Heyde" is typical, the instrument is adjusted until the subject viewed in the meter appears to lose its shadow detail. In the well-known "Justophot" of E. Meyer the hand-setting of the instrument is controlled by the moment of appearance of a small numeral as a bright figure in the centre of a dark field. (0. WH.) Developing and Printing Apparatus.—The apparatus used for producing finished prints from exposed sensitive materials differs according to the conditions under which the work is to be done. An amateur photographer who develops and prints his own photographs uses relatively simple apparatus. A portrait or com mercial photographer is equipped to deal with larger sizes and to produce a larger output, while a trade finisher who develops and prints large numbers of films exposed by amateurs uses quite elaborate apparatus.

Plates and flat films are generally developed by amateur work ers in flat trays, roll films in a special developing tank in which the film is held in the convolutions of a roll of celluloid which can be immersed in the developing solution. Sometimes plates are placed vertically in the grooves of a tank or of a rack which can be placed in a tank.