The Development of the Negative in

image, developer, times, dilution, bath, fog and density

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We have already learnt that the speed of development of the image proper decreases progressively. The rate at which fog grows also falls off, but less rapidly, with the result, in development, that the production of fog quickly overtakes that of the image proper.

Owing to the manner in which fog is formed in the image, the contrasts begin to decrease as soon as the fog has reached a certain value. Advantage is taken of this peculiarity to correct the excessive contrast which development of an under-exposed negative tends to produce. The use of a bath without bromide or with very little bromide, and to which an excess of alkali (preferably a caustic alkali) has been added, favours the rapid formation of a fog, which, however, tends to mask the less pronounced details of the image.

341. Effect of Dilution of the Developer. Slight dilution of the developer affects the speed of development and the formation of fog (§ 339). Considerable dilution may affect the character of the image to some extent.

In the case of a developer which is not directly oxidized by air (glycin developer) identical results are obtained by developing two iden tical exposures, one in a concentrated bath and the other in a portion of the same bath diluted twice, three times, . . . ten times, etc., if in the second instance the time of development is twice, three times, . . ten times, etc., that in the strong bath.

When the developer is directly oxidized by air (and this is the case with all developers except glycin), a certain fraction of the developer is oxidized by the air dissolved in the water used to dilute the developer, and the concentration of the developer is therefore somewhat less than denoted by its degree of dilution. For this reason the equivalent times of development must he longer than those calculated propor tionally to the degree of dilution. For instance, the highly concentrated commercial prepara tions of paraminophenol, which are usually used at a dilution of I: 20, if diluted to i : roo (i.e. diluted a further five times), may require times of development from six to nine times as long, according to the degree of freedom of the water from air (B. T. J. Glover, 1923). Condi

tional upon always developing to the same gamma (§ 202) it seems that the inertia of the emulsion (§ 337) is not influenced by dilution (K. Tchibissof and V. i'cheltsof, 1929).

In the use of a greatly diluted developer, 2 the active products which penetrate the emulsion are rapidly exhausted in the superficial layers and are only replaced in the fihn with extreme slowness. In these circumstances, the image of the shadows, formed chiefly of grains at a slight depth (assuming that the light has acted on the free surface of the emulsion), where the developer has exhausted itself only slightly, may be quite completely developed whilst development is still incomplete in the image of the high-lights, distributed throughout almost the entire thick ness of the film, for in regard to this latter the developer exhausts itself at the surface or at least takes up there an appreciable quantity of bromide before penetrating lower down (J. Sterry, 190o).

Advantage may be taken of this peculiarity to correct, to a certain extent, the effects of under-exposure, and, in a general way, to reduce the contrasts of an image. I But the full effect of this corrective will be obtained only by using a bath without bromide and by stopping development at the moment when the image of the shadows ceases to gain appreciably in density. If development were continued long enough, the image of the high-lights would, in fact, reach the same density that it would in a concentrated bath. A very dilute developer does not produce more detail than a concen trated one, but it clogs up the high-lights less easily, owing to the fact that it exhausts itself in these parts of the film. If the image has not reached the desired degree of contrast when the shadows cease to gain density, it is better to stop development and intensify the image afterwards, thus increasing all the densities proportionally rather than to continue develop ment, which would only increase the density of the high-lights without increasing the already insufficient density of the shadows.

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