Figure 21.2 illustrates the arrangement of a condenser enlarger. The purpose of the condenser (C) is to illuminate the negative evenly and to use the light
Conjugates of condenser
Figure 21.2 Optics of condenser enlarger output of the lamp as efficiently as possible. These aims are achieved if the condenser forms an image of the light source (S), within the enlarging lens (L). The latter is employed to form an image of the negative (N), in the plane of the easel (E). The negative causes some scattering of the light, so that a cone of light spreads out from each image point, as indicated by the broken lines in the figure. This cone fills the lens
(L), but very unequally, the intensity of the part of the beam that passes through the central area of the lens being much greater than that of the part that passes through its outer zone. One result of this is that stopping down the lens of a true condenser enlarger does not increase exposure times by as much as might be expected but tends to increase contrast, as it cuts down the light scattered by the negative.
The required diameter and focal length of a condenser enlarger depend on the size of the largest negative to be used. The diameter must be at least equal to the diagonal of the negative and the focal length should be about three-quarters of this value. The choice of focal length for the enlarging lens is restricted by a number of practical considerations. If it is too short in relation to the negative size, it will not cover the negative at high degrees of enlargement. If it is too short in relation to the focal length of the condenser, the light source must be a long way from the latter in order to form an image of the source within the lens. This necessitates a big lamphouse. It also means that the area of negative illuminated by the condenser is reduced. If, on the other hand, the focal length of the enlarging lens is too great, the distance from lens to baseboard is inconveniently long when big enlargements are required. The best compromise is usually achieved by choosing a lens
with a focal length at least equal to that of the condenser but not exceeding it by more than one-third. This results in a lens of focal length equal to or slightly longer than that of the 'normal' camera lens for the negative size covered (Figure 21.3).
Figure 21.3 indicates that any negative size which fits into or is smaller than the circular image field of the lens can be enlarged with that lens. Although this choice of focal length ensures even illumination, and a 24 x 36 mm negative could be enlarged with an 80 mm focal length enlarging lens, this would result in the enlarger head being raised higher than if a 50mm lens were to be used (see Figure 21.4). This may be inconvenient, and it may be impossible to obtain a sufficiently high degree of enlargement. In Chapter 4 we saw that the simple lens equation could be written in the form:
where v is the lens-to-baseboard distance, f is the focal length of the lens and m is the magnification.
Thus in Figure 21.4, if the degree of enlargement is x4 (i.e. m = 4), we can see that the lens-to-baseboard
Production of hard copy 351 Table 21.1 Negative contrast and type of enlarger
Condenser enlarger Diffuser enlarger
Contrast index G
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