436 HANDBOOK OF PHOTOGRAPHY 



developed to a contrast or gamma of less than unity so that the printing papers to be 

 used with miniature camera negatives have a natural contrast somewhat greater than 

 would be required for a "normal" (gamma equals unity) negative. 



Furthermore the absolute value of the exposure is a function of the paper used 

 (as some papers are more sensitive than others) and upon the degree of enlargement. 



Test-strip Method of Determining Exposure. — If the photographer eliminates all 

 variable factors in the process of development (by always developing completely), 

 then the final result depends entirely upon choice of paper contrast and upon the 

 exposure. To determine experimentally the correct exposure time a test strip of the 

 paper to be used may be exposed to the focused image for a time judged to be correct. 

 When developed, this print maj^ be inspected carefully to determine if more, or less, 

 exposure is desirable. 



The portion of the image chosen for the test should be a high light, perhaps the 

 brightest important portion of the image. Correct exposure will be that which will 

 produce a just discernible density after development. If the contrast grade of paper 

 chosen is correct, then the darkest shadow of the scene as represented by the negative 

 will be the darkest attainable color in the print. If the contrast (density range) of the 

 negative is greater than the printing paper will reproduce — ^this is frequently the 

 case — the photographer must decide which end of the scale to sacrifice, i.e., some of 

 the high lights or some of the shadows. 



The correct exposure will give a just perceptible deposit of silver in the brightest 

 high light. If normal paper is* being used and if the first exposure is too short, double 

 it for the second test. For contrast • grades of paper, increase the exposure about 

 30 per cent for the second test. 



Exposure Meters. — Various means have been worked out for avoiding the test-strip 

 method of determining correct exposures. If onh^ the correct exposure is desired, any 

 means of estimating the average transmission of light through the negative will aid in 

 arriving at the exposure. But if the correct contrast grade of paper is to be deter- 

 mined, then the photographer must have means for measuring the transmission of the 

 most and the least dense portions of the negative. 



One means that has been developed utilizes the photoelectric type of exposure 

 niet«r, so useful in estimating negative exposures. 



An ordinary printing box or contact printer is used as the light source. It may be 

 necessary to put a photoflood bulb in it to get sufficient light for measurement pur- 

 poses. A mask is cut for the top of the contact printer. In this is cut a small hole, 

 ^-i in. in diameter for example. Over this hole is placed the portion of the negative 

 to be tested. The photoelectric exposure meter is then placed on top of the negative 

 so that the light getting through the hole and the negative shines on a portion of the 

 sensitive disk near the edge. A greater reading will be obtained in this way than if a 

 spot near the center of the disk is chosen. The printing light is turned on, and a 

 reading of the exposure meter is taken. Then a reading without the negative is taken. 

 The ratio of the reading with negative to the reading without negative will give the 

 transmission. If a reading of 250 is obtained without the negative and 25 with the 

 negative, the opacity is 10 and the transmission is 0.1 or 10 per cent. Of course, 

 the scale on the exposure meter must be calibrated in numbers which are proportional 

 to the light intensity if this method is to be satisfactor^^ 



To determine the grade of paper that should be used, it is necessarj'- to make two 

 measurements, one of the transmission (or opacity) through the densest portion of the 

 high light desired to be correctly reproduced and one through the thinnest portion of 

 the shadow that is to be correctly reproduced. The ratio of these two transmissions 

 (or opacities) will give the exposure range required if the paper is to reproduce the 

 desired range correctly. As an example, suppose that the thinnest portion of the 



