EXPOSURE AND EXPOSURE DEVICES 259 



method. The method is especially suitable for auxiliary use with the darkest-object 

 method. 



Make a measurement of the same or similar object in the sunlight or other bright 

 location where a readable indication can be obtained on the meter. Then replace this 

 object with a sheet of white paper or white handkerchief and make a second reading on 

 this. The ratio of these two readings will give the ratio of the reflectance of the paper 

 to that of the object. Remove the paper, and place it in the position of the object 

 whose reading is desired (and which is below the reading of the meter) and make a 

 measurement from the paper. Divide this reading by the ratio previously formed to 

 obtain the light value of the desired object. 



For example, suppose the reading obtained on a similar object in a brighter location 

 is 2 and the reading on the paper or handkerchief in the same location is 20. The 

 ratio of these readings is 20:2 or 10. If now we obtain a reading of 1 from the sheet of 

 paper placed upon the subject whose light value is desired, the reading from the desired 

 subject will be Ho- It should be realized that this method is in certain cases a con- 

 venient approximation. It is most successful where no appreciable differences of 

 color between the two objects being measured are incurred. For example, if the light 

 is to be determined for dark-green foliage, it is best to make the ratio test between the 

 paper and brightly lighted foliage, rather than between the paper and, let us say, red 

 flowers. 



Incidentally, these methods of using the photoelectric exposure meter are equally 

 applicable for those cases in which a visual exposure meter is employed. 



Coordination of cameras and meters is of considerable importance in order to 

 obtain correct exposure. No meter or camera is perfect, and the calibration errors in 

 both will probably, bear an inverse ratio to the purchase price. This statement is 

 based upon tests made on many meters and cameras. Unfortunately it is not as sim- 

 ple to test and readjust an exposure meter or camera as it is to test and readjust a 

 watch, however, if with a particular meter or camera you obtain consistent under- 

 exposure or overexposure, the effect whether due to the camera or meter can be cor- 

 rected by changing the film-speed setting. For example if consistent underexposure is 

 obtained when using a film or plate rated at 23 Scheiner or 24 Weston then the rating 

 should be decreased to, perhaps, 20 or 21 Scheiner or 16 or 20 Weston depending upon 

 the extent of underexposure. This method of correction does not necessarily mean 

 that the film or plate is incorrectly rated, but the method of changing the speed ratings 

 merely offers a simple way to correct for incorrect exposures which may be due to 

 incorrectly timed shutters, incorrectly calibrated diaphragms, or for the type of 

 developer used, etc. « 



The actual aperture value depends upon whether the diaphragm is being opened 

 or closed. For example, tests on many cameras showed that the /-stops not only 

 varied in actual calibration but were dependent on whether the /-stop desired was 

 approached from a smaller or larger opening. For example, if the diaphragm was 

 moved from //3.5 to //8, the aperture would be larger than if the diaphragm was 

 moved from //16 to //8. These errors are larger on short-focal-length cameras such as 

 miniature and cine cameras than on the long-focal-length cameras because of the much 

 smaller actual aperture. It was also found that these errors were considerably greater 

 in the low-priced cameras. It is therefore recommended always to set the /-stop 

 from the same direction, which will result in the same aperture at all times for the same 

 /-stop calibration. 



When using an exposure meter, it should be appreciated that it is a photographic 

 tool and like any tool the more knowledge of it and the more experience you have with 

 it, the better it will serve you. There has been and probably will continue to be much 



