These are of very many different designs, and it is not possible here to deal with them all, but the most general design will be taken in each case. A portrait photographer will use a heavy, rigid stand camera, most amateurs use a folding type of hand camera, combining portability, lightweight, and easy operation, while a professional press photographer will use a hand camera which is specially adapted for instant use, with maximum view finding capabilities. In all types, however, the vital action is the same, i.e., that the image of the object or objects viewed by the lense is brought to a focus on a prepared plate or film in the light-tight camera body.

Lenses

These are used to bring the image of the object, etc., being photographed to a focus on the sensitive plate or film. Now a ray of light travels in straight lines in air for instance, but on passing from air into glass its course is altered, or refracted. If now parallel rays of light pass into a lens, whose surfaces are parts of spheres, and which is thicker in the centre than at its edge then these parallel rays are brought to a point on the other side of the lens, these are called converging lenses. This point is the focus of the lens, and the distance of this point from the lens is termed the focal length. Other lenses may be thinner at the centre than at the edges, the surfaces still being spherical, and such are termed diverging lenses. Parallel rays of light striking a diverging lens are not brought to a focus but diverged outwardly, such a type of lens is used in some compound photographic lenses. Of course, relatively few rays of light entering a camera lens are parallel since as the object being photographed is nearly always larger than the lens diameter, it follows that the greater number of light rays converge towards the lens. Again the rays of light passing through the lens at the edges are not brought to a focus at the same distance from the lens, as rays passing through the lens centre, and therefore do not focus sharply on to a screen. This trouble is called spherical aberration, and can be reduced by a form of compound lens, or by the use of a stop or diaphragm, so that a certain amount of the lens at the edge is not used, the smaller the stop the more the rays are confined to the lens centre. Another difficulty occurs due to the fact that ordinary white light consists of a combination of certain colour rays, and unfortunately these different colour rays are not refracted equally, or in other words they are not brought to the same focus. Blue and violet rays give the least illumination, and are brought to a focus nearer the lens than other colours. This trouble is termed chromatic aberration. In addition, when we bring the object into apparent focus on the screen, the eye obtains this and judges the focus sharpness by the aid of other colour rays than those which strongly affect the plate or film sensitive surface. It has been discovered that different compositions of glass refract these different colour rays to different amounts, and that it is possible to build up a compound lens, by cementing special single lenses together, these latter being of different kinds of glass and of different shapes, this compound lens is then said to be an achromatic lens. If a single lens be used in a camera it will give excellent results for landscapes, etc., but is not suitable for architecture, etc., as in that case the use of a stop will cause distortion. It should here be pointed out that the expression ' single lens ' is used to cover what appears to be a single lens, though actually it may be made up of several pieces cemented together, lenses, however, separated by any appreciable distance and held in a metal or other case or tube are termed doublets. When this latter type is used for camera work it generally consists of a meniscus lens separated at some distance from a similar lens with the concave surfaces inward, stops or an adjustable diaphragm are fixed between these lenses, and the combination is termed a rapid rectilinear or symmetrical lens. The reason of these names is that the combination of the two meniscus lenses and the stops prevents distortion of the image of architectural or the like objects. It should be explained that a meniscus lens has a saucerlike appearance, one side being convex and the other concave, with the convex surface having a smaller radius than the concave surface, resulting in the lens being thicker at the centre than at the edges. The focal length of the lens used affects the size of the image or photograph obtained, i.e., the diameter of the image varies directly as the focal length of the lens. If using the same lens, the diameter of the image varies inversely as the distance between the object and the lens, i.e., an object photographed at 100 feet away gives half the diameter image of the same object at 50 feet distant, the area of the image varies inversely as the square of the distance, i.e., the image of an object at 50 feet distant is four times the area of the image of the same object if at 100 feet distant. From the above it will be seen that the size of the image depends only upon t he distance of the object from the lens, and upon the focal length of the lens. It is clear that lenses call for careful handling, and if need exists for taking them to pieces, then extreme care should be taken not to scratch, or damage the lens surfaces, or to cross thread any parts of the screwed tube or holder. An adjustable or Iris diaphragm is constructed of blackened thin metal plates so cut and mounted that on rotating a projecting knob or finger the central hole is increased or decreased in diameter, leaving always an approximately circular aperture. Such a diaphragm gives a ready means of increasing the sharpness of focus, and also of varying the exposure time.

Shullers

These in the early cameras merely consisted in a cap which was taken off the front of the lens, and replaced after the desired exposure, modern shutters give means of giving a time of exposure of any length upwards, and also downwards to say one thousandth of one second. The average camera shutter will work to one hundredth of one second or longer. Types are many and various, the cheap forms drop a metal shutter across the lens rays, and the speed of this shutter is controlled by the tension put upon it by an adjustable spring. More expensive types allow the ray path to be uncovered by pressing a small lever causing a shutter to expose the plate. This or another shutter or plate obstructs the light after a time determined upon its previous adjustment. One type times the length of the exposure by altering the travel of a small air dash-pot, and can work perfectly by this means between the limits of one second down to 1/100 second. Another type consists of a fabric roller blind which is caused to pass across the path of the light rays, in this roller blind is an adjustable opening reaching right across the shutter. If this amount of opening is adjustable, and also the speed of movement of the whole blind, it is obvious that a wide range of speeds, and up to a high speed can be obtained. Should any of these shutters go wrong, and stick in any way, their method of working will be found to be very simple, and obvious, if carefully taken apart, and the action considered. Any repair work will require fine tools and a light touch.

Bellows

These do not of course appear in all cameras, such as the box type, but in folding, etc., cameras, they are always used, since they provide a ready means of providing a light-tight space between the lens and the sensitive plate, etc. They are generally made either of thin leather or of imitation leather. They should be inspected now and then to see that they really are light proof, if any hole however minute be discovered, it must be patched in such a manner that the freedom of movement of the bellows is not hindered at all. A thin piece of leather, say wash leather, fixed to the inside by means of Chatterton Compound will be found to give a lasting and flexible repair. The places of attachment of the bellows to the camera body, and front, should also be carefully examined for any hole through which light can penetrate.