In the last article of this series, published in the June issue of Studio Light, we saw that we can represent the way in which the scale of tones which occur in a natural object are translated into the densities of the negative by a curve which is shown in Fig. 1. Throughout the greater part of this curve equal increases in exposure are represented by an equal rise in the densities and this portion of the curve corresponds to a technically perfect negative; that is, one in which the opacities of the negative are proportional to the light reflected by those portions of the original subject which they represent. But this proportion between the densities and the exposures does not hold throughout the whole of the curve in what is known as the period of under-exposure, and again at the end of the curve in the period of over-exposure. The straight line portion of the curve shows the capacity and the limit of a given material to render a scale of tone values correctly.

It is common knowledge that both the contrast and the density of a negative increase during development, and we may therefore ask what effect the amount of development will have upon this curve which shows the relation between the density and the exposure.

If we develop two plates for different times, one for three minutes, let us say, and the other for six minutes, we shall find that the two curves will be identical in shape, and in each of them the straight line portion corresponding to the region through which reproduction will be correct will cover the same range of exposures, but that the steepness of the two curves will be different, the curve of the plate developed for six minutes being much steeper than that of the plate developed for three minutes (Fig. 2). This means that as we continue development each density increases to the same proportional extent. We shall not find that our highlights gain density rapidly and then stop or that our shadow detail builds up first and then the highlights gain upon it, as some photographers have thought, but that an increase in development means a proportional increase in every part of the negative scale. If we add 50% to the density of the shadow detail we shall add 50% to the middle tones of the negative and 50% again to the highlights, and since each density increases in the same proportion we get an increase in the contrast shown between the highlights and the shadows. This contrast can be measured from the steepness of the straight line portion of the curves; that is, by the rise in density which corresponds to a given increase of exposure. If, with the units we have chosen, the rise of density is equal to the increase of exposure, then we can say that we have a contrast of unity; if for the same amount of exposure another plate gives twice as much density, we can say that its contrast is two; if it is three times as much, the contrast is three, and so on. (Fig. 3.)

Therefore, during development the contrast increases. At first it increases rapidly, then the rate of increase begins to fall off and the contrast increases more and more slowly until finally no increase in the time of development will make any difference and the plate has got to the point where it has reached its maximum contrast, the value of which depends upon the plate, but beyond which the contrast cannot be pushed by prolongation of development. In Fig. 4 we see a number of lines showing the contrast obtained with development of one minute, two minutes, four minutes, six minutes, eight minutes, and twelve minutes, and it will be seen that they become closer and closer, and if we develop for a much longer time we reach the limiting value, which is marked infinity, beyond which no amount of development will push the contrast on that material. If development be further prolonged, we shall only develop fog over the whole plate.

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Fig. 1.

The Photographic Rendering Of Tone Values III By D StudioLightMagazine1917 144

Fig. 2.

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Fig 3.

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Fig. 4.

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Fig. 5.

This limit of contrast obtainable depends upon the photographic material. High speed portrait plates have low values of contrast since no portrait requires to be pushed to a contrast exceeding unity. Plates used for landscape work and commercial photography have higher values and will give greater contrast on development. They develop more quickly and easily, give contrasts exceeding the maximum to which the fast materials can be pushed, while the greatest contrast of all is obtained with the special slow emulsions made for process work, where every effort is made to get the greatest possible contrast so as to get clear lines on a completely opaque field. The maximum contrast given by process plates is frequently as high as four, which means that if we have in the original subject two tones one of which is twice as bright as the other, then in the negative, the part representing the higher tones will transmit only one-eighth of the light of that corresponding to the lower tone. Thus, Fig. 5 shows us the curves of Seed 30, Seed 26, Seed 23 and Seed Process plates, each being developed to the maximum contrast available in prolonged development.

Although we cannot obtain a very contrasty negative upon an emulsion designed to give a maximum contrast yet we can obtain soft negatives upon a plate having a high maximum contrast by developing for only a short time. In practice, however, every photographer knows that if he uses a plate designed to give great contrast he will not get satisfactory portrait negatives upon it even if the time of development be short. The reason for this is that hard working plates also have a very short scale in the straight line portion, so that only subjects of very limited scale can be rendered on the straight line portion of the curve. A process plate, for instance, will be able to render a contrast of only 1 to 4 correctly as compared with the great range of 1 to 256 obtainable on the Seed 30 plate, so that if a process plate be used for portraiture, even if soft negatives be obtained by short development, the quality of the negatives will be very unsatisfactory.

Artura Iris Print, From Eastman Portrait Film Negative By Barnum Studio Cincinnati, 0.

Artura Iris Print, From Eastman Portrait Film Negative By Barnum Studio Cincinnati, 0.

We must next consider the relation of the contrast to which we develop the negative to the scale of the original subject. Suppose that we have a range of light intensities in our subject from 1 to 100. Then if we develop the negative to a contrast of unity and if the length of the straight line representing the quality of the material and the exposure are such that we get perfect reproduction of those 100 tones in the negative we shall have a negative in which the ratio of the highest transmission to the lowest transmission is the same as that of the subject; namely, 1 to 100. If this scale is too great for printing on the papers which are available, we can reduce the scale by lowering the contrast of the negative; that is, by developing the negative for less time, which will slightly reduce each tone in the same proportion. On the other hand, in the case of flat subjects, we can increase the available scale of the subject for the printing paper by increasing the time of development, thus increasing the scale of contrast in making the negative.

Provided that the contrast of the subject is not too great for the scale of the negative material and that the exposure is such that the scale of the subject falls on the straight line portion of the curve, then development to a contrast of unity will make the scale of intensities of our negative the exact inverse of the scale of intensities of the subject. (To be continued.)

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Artura Iris Print, From Eastman Portrait Film Negative By Barnum Studio Cincinnati, O.

Artura Iris Print, From Eastman Portrait Film Negative By Barnum Studio Cincinnati, O.