THE
CONTRAST
OF PHOTOGRAPHIC
PRINTING
PAPER.* t BY
LOYD A. JONES. Research Laboratory, E a s t m a n Kodak Company.
SECTION E. CORRELATION B E T W E E N SENSITOMETRIC CONSTANTS OF POSITIVE MATERIALS AND T H E CHARACTERISTICS OF THE OPTIMAL POSITIVES AND NEGATIVES.
A CASUAL inspection of the data relating to characteristics of the positive materials, the optimal positives, and the optimal negatives indicates the existence of certain more or less definite quantitative relationships. It seems worth while now to analyze these relationships more critically with the hope of acquiring a more thorough understanding of how to obtain the best possible photographic quality by choosing from a wide variety of developing-out papers available the one best adapted for the rendition of a part,icular negative. The general principles controlling the choice of a positive material to fit a given negative are quite generally recognized and understood in a qualitative way. For instance, the practical worker knows that a soft negative, one which has been developed for a relatively short time or in which the brightness scale of the object reproduced is short, must be printed on a contrasty or hard paper (that is one having a short exposure scale) in order to obtain a positive of satisfactory quality. On the other hand, if the negative is hard, due either to long development or to great brightness contrast in the object, the paper must possess a great exposure scale in order to yield a satisfactory result. It is apparent, therefore, that a more or less definite relation exists between the density scale of the negative and the exposure scale of the positive material yielding a satisfactory reproduction of the negative. An inspection of the data presented in the previous section of this communication indi* Communicated by Dr. C. E. K. Mees, Director of Laboratory, and published as Communication No. 264 from the Research Laboratory of the Eastman Kodak Company. l Continued from p. I56 of the January issue. VOL. I04, No. I2X9---4 41
42
LoYD A. JONES.
[J. F. I.
cates that there is not, however, a precise quantitative relationship between these two factors. As was pointed out in the previous section, the proportion of the total exposure scale of the positive material which is available for the rendition of the negative depends very much upon other factors. A careful study of the available data has shown some very definite systematic relationships between the various factors involved and in this section these relationships will be discussed in detail. I. G E N E R A L R E V I E W OF TONE R E P R O D U C T I O N T H E O R Y W I T H A P P L I C A T I O N TO A S P E C I F I C CASE.
A rather thorough knowledge of the principles of tone reproduction is required for understanding the significance of the results derived in this section. It may be well, therefore, to review briefly some of the principles of tone reproduction and to explain in greater detail the construction and interpretation of the reproduction diagram which was shown in Fig. 4, Section A (p. 199 ) . The diagram mentioned was drawn to represent a perfectly general case using hypothetical characteristic curves for the positive and negative materials. This will now be applied to a specific case in order to show the character of the tone reproduction obtainable on a positive material with a negative of optimal quality. For this purpose positive material No. 9 (Azo F hard) has been chosen. A complete diagram, showing the tone reproduction obtained on this material when used with the negative which was found b y the statistical method to be of optimal quality, is shown in Fig. 52. On the line O X a suitable scale of object brightness, log Bo(p), is established. Referring to Section B, Table V I I I (p. 483), it will be found that the following values were determined relative to the object: Maximum photographic brightness, B0 (p) - 3 . 4 o . Minimum photographic brightness, B0(p) - 1.95. Photographic brightness scale of object, B So(p) ~ 1.45. Maximum visual brightness, B0 (v) = 3.4o. Minimum visual brightness, B0 (z') - 1.81. Visual brightness scale of object, BSo (v) = 1.59. It will be noted that the brightness scale as measured visually is somewhat greater than when measured photographically. This is due either to lens flare or to the colors present in the object photographed. The lowest brightness found in the object was