In general, all enlargers are similar in design and operation. They have an enclosed light source, some method of providing an even distribution of light over the negative, a negative carrier, a lens, a means of adjusting the lens-to-negative and lens-to-paper distances (fig. 11-7). The degree to which the image is enlarged can be referred to in terms of diameters; for example, a two diameter or 2X enlargement is twice the length and twice the width of the negative image, or four times the area A three diameter or 3X enlargement is three times the length and width of the negative image, or nine times the area.

Most enlargers have a tungsten lamp as a light source. The lamp is enclosed in a lighttight housing that is ventilated to prevent excessive lamp heat from damaging the negative. Some enlargers have blowers to circulate air and cool the inside of the lamp housing.

The negative carrier used in an enlarger may be either a dustless type or a glass sandwich type. The dustless type of carrier is made of two metal plates with an opening in the center large enough to hold the negative. The negative is placed between these plates and held in position by its edges. This type of carrier is good for negatives 4x5 or smaller, since these negatives are stiff enough to remain flat. The glass sandwich type

Figure 11-8. Condenser enlarger.

of carrier is a holder where the negative is placed between two sheets of glass. This type of holder is used for larger negatives since they have a tendency to sag in the center when they are not supported by glass.

The lens used in an enlarger should have an angle of field large enough to cover the negative being printed. A lens with a focal length approximately equal to the diagonal of the largest negative to be printed provides sufficient angle of field.

The bellows of an enlarger should be capable of extending at least twice the focal length of the lens. This amount of bellows extension is necessary for making 1:1 reproductions. Although it is possible to make reductions to any desired size, the bellows on most enlargers cannot be extended far enough to make reductions smaller than 1:1. Smaller reductions can be made by using a longer focal-length lens, but a better method is to use a reducing attachment. A reducing attachment consists of a section of supplementary bellows fitted with a longer focal-length lens.

The systems used to distribute the light evenly over the negative divide enlargers into three general types condenser, diffusion, and condenser-diffusion enlargers.