Every photographer is aware that the printing paper must be adapted to the negative used. It is hopeless to attempt to print a strong contrasty negative upon a hard paper made for printing from weak negatives, while on the other hand a weak, flat negative requires a strong printing paper to obtain prints of sufficient snap and contrast. In order to understand this relation between the printing paper and the negative let us consider the way in which printing papers behave when given a series of increasing exposures to light.

We can try the effect of an increasing series of exposures upon a printing paper in exactly the same way as upon a plate, that is, we can give a first exposure just sufficient to get a barely perceptible image after development, then expose another portion for twice the time, another for four times, and so on. We shall get on a strip of paper the effect shown in Fig. 1. Now instead of measuring the light transmitted by the various densities, as we did in the case of the plate, we must measure the light reflected from them. We get a series of "reflection densities" on paper corresponding to the transmission densities of the plate and we can express the result in the form of a curve just as we did in the case of the plate.

Fig. 1.

Thus in Fig. 1 we see that the densities increase gradually at first, as shown on lower portion of the curve, then grow in equal steps for equal increases of exposure, as with the plate, and then the increase not only grows less, but very soon stops altogether, as shown by upper portion of the curve. This result only occurs with a plate with very great exposures indeed. Therefore, a paper is seen to differ from a plate in that we rapidly reach a point where we have obtained the maximum blackness of deposit which the sensitive emulsion is capable of giving and where no further increase of exposure will enable us to obtain a more intense black.

The reason for this is that with the paper we are dealing with reflected light, and not with transmitted light, as in the case of the plate, and the light is reflected from three surfaces - from the surface of the gelatine, from the surface of the silver deposit and that which is not absorbed in passing through the silver deposit is reflected from the paper beneath. The effect of the surface reflection of the gelatine can be seen very readily if we measure the maximum black obtainable with different surfaces of paper. A glossy paper (viewed at such an angle that the surface does not reflect light directly into the eye like a mirror) has much the deepest black, the strongest obtainable deposit on such a paper reflecting only 2% of the light falling upon it. A velvet surface will give a less intense black, reflecting about 4%. A matt surfaced paper will give a still less intense black, most matt papers reflecting about 7% of the light from the deepest shadows obtainable. Now the total scale of contrast obtainable from a paper depends upon the ratio of the light reflected from the white paper (taken as 100%) to that reflected from the blackest deposit obtainable, so that this scale of contrast will vary with the surface of the paper. With matt papers it will be 1 to 15, with velvet papers 1 to 25 and with glossy papers as much as 1 to 50.

Fig. 2.

In this scale of contrast the eye can discern a certain number of tones. Let us consider a strip of paper giving a complete range of deposits from white paper to the maximum black obtainable on the paper such as can be obtained from a series of increasing exposures (Fig. 2).

How many distinct tones can we see in such a strip? With velvet papers the eye can detect about 100 tones, with glossy papers as much as 150; perhaps 100 tones which can be definitely detected as distinct steps would be a correct average for printing papers in general. Now by the scale of a printing paper in exposure units we mean the range of exposures which will give all these hundred tones; that is, if an exposure of one second will just give the first perceptible difference from white paper showing the first trace of tint on the paper, and an exposure of twenty seconds will give the deepest black of which the paper is capable so that no increase of exposure will produce any denser black, then we may call the scale of the printing paper 1 to 20.

Fig. 3.

Different printing papers have different scales of exposures to fit the negatives for which they are intended. Thus the Velox papers, intended for amateur negatives which tend to flatness, give their whole scale of tones with a short range of exposures. An exposure of from one to twenty units will give the whole tone range of Special Velox, from one to ten that of Regular Velox and from one to five that of Contrast Velox, so that with this last very "contrasty" paper we get the maximum black which the paper can give with only five times the exposure necessary to just produce a tint (Fig. 3). The "contrast" of a paper, in fact, chiefly depends upon the "scale" of exposures which will give the full range of tones from white to black.

The "scales" of papers intended for portraiture, where the negatives are of good quality and have a full scale of gradation, are necessarily greater than is the case with the Velox papers intended for amateur printing. Thus the scale of Artura Iris is 1 to 60.

It must be remembered that the scale of exposures required to give the full range of deposits on a given paper is not an indication of quality but only of the contrast of the negative to which that paper is adapted. It is sometimes suggested that a good paper will give a full range of tones. But all papers will give a full range of tones from white paper to the deepest black of which they are capable. The requirement for quality is that they should give this range evenly. In Fig. 4 we see the way in which two different papers give their scale of tones with varying exposure. Both give the same range of tones, both require the same range of exposures to give the entire scale, that is, both have the same scale, but in the one the deposit grows evenly with the increase of exposure, while in the other the curve is not straight at all. The deposit at first grows more and more rapidly and then later gains less and less rapidly until the maximum deposit is attained. The paper showing the even growth of deposit has good "quality," that with the uneven growth, although it gives the same maximum black and the same scale, has poor quality.

Fig. 4.

Bromide Enlargement, From Seed Graflex Negative.

From "Joan the Woman" Directed by Cecil B. De Mille.

For papers, therefore, as well as for negative making materials, quality depends upon the proportion of the curve which is a straight line, and the straighter the curve, the better the quality.

When the curves of a number of papers were measured it was found that, just as Seed 30 plates stand out among negative making materials by reason of the length of the straight line portion of the curve, so Artura Iris paper is distinguished by its straight curve, marking an even range of tones throughout its entire scale.

In the next article of this series we shall consider how the scale of the printing paper must be related to the negative. (To be continued.)