Fig. 42 represents the section of a correctly exposed plate, with short, medium, and long development. The difference in the lines A B c, is the shallowness or steepness of the contrast obtained by the varying time of development. The negative having had correct exposure the half-tone is in the centre in each case, although the outer tones vary considerably; A, B, and C might all be good negatives for different purposes.
A. - A soft negative with little contrast, suitable for enlargements on bromide paper of a large size.
B. - A good all-round negative.
C. - A negative with more contrast, suitable for carbon printing.
Altering the strength or the composition of the developer is equivalent to altering the time of development, which would produce exactly the same effect.
The development of either an under-exposure (Fig. 43) or an over-exposure (Fig. 44) proceeds in exactly the same way as a correct exposure; that is, each tone will develop in the ratio decided by the exposure. If development is continued long enough to double the deposit on the shadow, the deposit on the high light and the half-tone will be doubled also.
Fig. 43 shows three stages in development of an underexposed plate: A, development for a short time; B, the development continued until the deposit in the shadow has been doubled; C, continued development until the deposit on B has been doubled; and in each case the deposit on the highlight and half-tone has also been doubled. The exposure having placed the half-tone much nearer to the shadow, it remains in that position, whether the negative is flat as in A, or contrasty as in C.
Fig. 44 shows the same three stages of development in an over-exposed plate; only that in this case the half-tone has been placed by the exposure nearer to the high light. We get practically the same result in both cases, with the exception that the displacement of the half-tone is towards the shadow with under-exposure and towards the high light with over-exposure.
In Figs. 43 and 44 the development has been carried on so that the density of the shadow and high light may correspond to those of correct exposure (Fig. 42); but it will be seen that in both cases the half-tone is untruthfully rendered. And when the half-tone has been placed in this false position it remains there in whatever way it may have been developed. The only control the photographer has is to decide how far he will carry development, and by this means he can determine the amount of contrast obtainable in the negative. This degree of contrast should be regulated according to the medium to be used for printing; some processes require a soft negative, while others require one with more contrast.
The density of a negative has very little effect on the printing value, except to increase the time actually occupied in printing, whereas the contrast is the governing factor for a good print. Developing negatives to a certain density is a fallacy, although photographers have been invariably taught that this is the correct method. It is far better to develop for a certain contrast. Many have the idea that density and contrast are synonymous, whereas they are totally different; the density of a negative is practically the thickness of the deposit, while contrast is the distance between the high light and shadow. It is possible to have a dense negative with little contrast, or a thin negative with plenty of contrast. If four plates of the same batch were exposed on a subject each receiving a different exposure (within the latitude of the plate) and developed together in the same developer for the same length of time, the result would be four negatives of different density, but all would have the same contrast. They would all give equal prints, but the time of printing would vary according to the density. The negative which received the longest exposure would be denser than the others and take longer to print. On the other hand, if these negatives were developed in the dark room, and development had been stopped when each reached the same density, they would all have different contrasts. And in this case it would be impossible to get equal prints. Some negatives would give flatter prints than the others, although each would take about the same time to print.
Therefore the exposure does not only decide the relations of the tones, but the density of the negative, and the development decides the contrast between the tones. And as the determination of this contrast is the only means of control the photographer has, there is no necessity to watch the process of development, which can be just as well carried out by calculation.
The necessary factors are a standard developer, a standard quantity of water, a standard temperature, and a standard time. Any variation in the first three factors can be corrected by altering the last. A strong or a warm developer acts more quickly than a cold or weak one, but this can be corrected by altering the time of development. The amount of contrast is also regulated in the same way, so that everything required can be obtained by altering the time of development.
Time development can be carried out in an open dish; but a tank is far preferable, as the dark room need only be used for loading the tank. With films and the Kodak developing tank, the dark room can be done away with entirely, as this system allows of the film being placed in the developer while the photographer is in ordinary daylight.
If all the factors are carefully adjusted when developing in the tank by time, the best possible result will be obtained, whether the exposure has been correct or incorrect; and this system entirely removes the temptation to over-develop under-exposures and to under-develop over-exposures, which is usually done when development is carried out by ocular observations. With under-exposures photographers continue the development, hoping to get more detail in the shadows, and make the negatives so hard that they are spoilt and will only yield prints of a soot and whitewash character. Over-exposures are dense and opaque in the high lights long before development is complete. Hence there is a tendency to take them out of the developer too soon; and this practice is so general that it is usually understood that the thin negative is the result of over-exposure, whereas it is due to under-development. Errors in development are often confused with errors in exposure : which leads to doubt where the fault really lies. But when all negatives are developed for the same time, and the other factors kept normal, there will be no error in development. Consequently, the errors in exposure are easily located, and time development teaches correct exposure in the simplest way.
Nearly all developers are available, but it must be remembered that variations of temperature have not the same effect on all. The normal time is usually calculated for 65° Fahr.; the time given must be increased by one-half in the case of pyro, and about doubled with rodinal, metolquinol or glycin if the temperature of the solutions has fallen to 550. The Kodak formula is as follows:
A. Pyrogallic Acid....... 1 oz.
Sulphuric Acid....... 20 minims.
Water........ 28 oz.
B. Sulphite of Soda (cryst.)..... 6 oz.
Carbonate of Soda (cryst.) . . . 4 „
Water.......to 28 „
For development take 1 1/2 oz. of each and make up to 20 oz. with water. Time of development is 10 minutes at temperature 650 Fahr.
Glycin is a great favourite, and Fuerst's formula gives a very clean negative, allowing for reasonable latitude before the half-tones become unprintable.
Hot water ......... 10 oz.
Sodium Sulphite...... 1 1/2 ,,
Potassium Carbonate...... 3 ,,
Glycin......... 240 gr.
Use a 20 oz. measure in mixing, and add the glycin slowly to prevent excessive effervescence. For use make each ounce up to four ounces with water. The tank must be carefully washed before use with this developer, as any trace of hypo will produce bad stains. An alternative is the metolquinol formula in the last chapter. Some of the original advocates of time development advocated rodinal and similar formulae so diluted that twelve or more hours were required; in fact, that plates should be put into the tank overnight and found in the morning correctly developed. Such prolonged development has no good effect on the gelatine of the plate, and will often result in silver fog. For the benefit of experimenters we give Bothamley's formula for one-hour development at 650.
Sodium Sulphite..... 46 gr.
Developing and Fixing Combined. (V. Cremier).
Sodium Sulphite......150 gr.
Hypo (20 per cent. solution, 4 oz. to the pint) . 2f drams. Water.........3 1/2 oz.
Fixation is said to be complete in about fifteen minutes, but with many varieties of plates a second fixing bath will be necessary and may be given in daylight.