MEASUREMENT OF OPACITY Hurter and Driffield define opacity as "the optical property of a substance to impede the passage of light through it." On this definition the author of the present work suggests the following practical method for measuring and defining that property. In the sense of the def inition it is seen that even clear glass has opacity, since, to an extent, it impedes the passage of light through it. A negative impedes the light by reflection, from both the outer and inner surfaces of the glass, as well as by absorption and diffusion, yet the total amount of hindrance to the light is comprehended in photography by the one word "opacity." Should a tinting medium be exposed to any light for one second and a tint be secured it may correctly be considered that the clearness of the atmosphere, i.e., its degree of opacity, in a sense determines the depth of the tint secured. Had the atmosphere been smoky instead of clear it is evident that the same tint could not have been secured in the same time. If for example the time required to obtain the same tint through the smoky atmosphere be five times that required through the clear atmos phere then it might very properly be said that the opacity of the former was five times the latter.
Opacities in general then, as those of the different planes of a negative or that of any substance not entirely opaque, may be con veniently referred to that of the ordinary clear air, the opacity being proportional to the time required to produce any given tint through the medium as compared to that required to produce the same tint through the air alone. If for ex ample it required 8 seconds to secure a tint through some particular part of a negative which was secured in 2 seconds without the negative (through the air alone) then that plane of the negative measured may be said to have a rel ative opacity of 4. It must be noted that the opacity under consideration is that for the special kind of radiation used in photography. Thus some piece of deep blue glass may be optically quite opaque and yet have very low actinic opacity.
Comparative exposures for finding the relative opacities of different parts of a negative are conveniently made in an ordinary printing frame with a piece of black paper, having a small hole in it, placed between the plane to be measured and the tinting medium. By this means the latter may be examined from time to time as in ordinary printing in order that the tint secured may be made to match the temporary standard tint first obtained through the air. This tem porary standard tint is very conveniently that produced by a one second exposure to direct clear sunlight or by 2 or 4 seconds to an open sky, as found necessary to produce some vis ible tint.
It is a fact easily verified by measurement that the clearest possible area of a glass plate negative, or of a film negative when measured with the glass upon which it is laid for printing, although consisting only of a layer of clear gelatin and its glass support has an opacity about twice that of clear air. The opacity grad ient of a negative therefore never starts from zero.
Having measured the thinnest and thickest parts of a negative its contrast factor may be found by dividing its greater opacity by the less.
Should it be desired to obtain only the contrast factor between the two extremes of a negative, without actually measuring the several opacity planes then the temporary standard tint, the one to be matched, may be taken through the thinnest part of the negative instead of through the air alone. When this tint is matched through the thickest part of the negative the ratio of the corresponding tinting times will be the con trast factor of the negative measured.
It may be observed that the negative from minimum contrast subjects and from all exterior subjects are considerably more opaque at their thinnest areas than negatives taken in interiors, as portraits, etc. There are seldom seen in negatives of the former group, areas consisting of the so-called "clear glass" parts. This is owing to a greater general distribution of light throughout all parts of exterior subjects and to excessive reflections within the bellows. This explains why exterior negatives should usually on the whole be more opaque than those taken in interiors in order to obtain a desirable tone scale, as of course, since the thinnest parts are of considerable opacity, the thickest ones must be of proportionally greater opacity to secure the usual contrast.
The importance of making a negative agree in its opacity contrast with the degree of hard ness or softness of the printing paper or other medium has already been emphasized (pp. 15, 94). By making three prints from a medium negative, one on a very hard or contrast paper, another on one of normal gradation and another on a soft paper this truth should be fully appre ciated and will need no further demonstration.
According to the chart on page 109 it is seen that the straight part of the curve includes the exposures of from 8 or 16, to 1024 inertias according to the present author's method of con sidering exposures. In explaining the charac teristic curve Messrs. Hurter and Driffield explain (p. 108) that thickly coated, slow plates were used. However accurate it may be theo retically, that a one thousand inertia exposure of an emulsion is the maximum correct limit, this is certainly not true in practice with the emulsions generally in use, which, as I have stated, begin to show a dulling over of surface texture in the image when an exposure of 64 inertias has been given, even with the softest working emulsions now employed. It seems to the present author that the observation of the characteristics of developed spots of emulsion which have been exposed by direct illumination, as from a flame, cannot give the information which is required to enable us to use these emulsions for the purposes of practical photog raphy, since the consideration of texture rend ering does not enter and since it is here that the effect of over-exposure is first looked for in actual practice. For this reason the "inertia ex posure" method of considering the character of emulsions (p. 83) is recommended as the most correct and useful from the practising photog rapher's standpoint.