EXPOSURE AND EXPOSURE DEVICES 



251 



corresponding numeral or letter. The dimmest figure which can be read is taken as 

 the appropriate "light value" for the conditions under consideration. The outer 

 telescoping tube is usually provided with a series of scales which relate the light values 

 as seen through the instrument, the film speed, the shutter speed, the aperture, and, 

 sometimes, filter factors. The proper exposure may usually be determined with a 

 simple setting of a ring which may be turned or slid relative to the fixed scales attached 

 to the main tube. 



Since the operation of this type of instrument depends upon a subjective estimate 

 of the dimmest figure which can be read, it is necessary that the sliding tubes 

 be adjusted for proper focus and that the eye become 

 accommodated to the light at which readings are taken. 

 It is usuallj^ recommended that no readings be taken 

 until the operator has looked through the exposure 

 device for at least 5 sec, allowing this time for eye 

 accommodation. 



These visual exposure devices are simple, inexpen- 

 sive, small, convenient in operation, and robust. They 

 are usually superior to photoelectric devices for low 

 values of light intensity where the ordinary photoelectric 

 devices are insensitive. A properly made and calibrated 

 and intelligently used visual exposure meter will find use 

 in almost any photographic activity. The principal 

 disadvantages of these instruments are that they are 

 subjective in operation, thereby introducing human 

 errors, are relatively slow in affording a reading, and 

 some types are improperly calibrated. 



The relative calibration of several visual exposure 

 meters was determined from the calibrations attached 

 to the meter, and it was found that at least some of 

 these visual meters are poorly or carelessly designed. A 

 light value as determined by the meter might give a 

 correct exposure for one aperture and shutter speed, 

 but the same light value might be off several hundred 

 per cent when used with different apertures or shutter 



speeds. This error occurs in the printed scales attached tographer to obtain a goodldea 

 to the instrument and is in addition to any errors of of what his photograph will 

 absolute exposure which may result from incorrect look like and at the same 

 manufacture of the stepped wedges. It may be ex- ti°^« ^^^^^les him to deter- 



, , -n , r • e mme exposure. 



pected that some error will occur when fractions of a 



second or minute are rounded off to simple rather than awkward values. Such errors 

 should not exceed 25 per cent, however, and a 25 per cent error is negligible for most 

 photographic purposes. 



Table XXV shows the values of relative exposure for various apertures and the 

 exposures for these apertures as indicated by several popular types of visual exposure 

 meters. It was arbitrarily assumed that the exposures were correct for an aperture 

 of //2.8. This was the largest aperture common to all instruments, but any other 

 aperture might be considered as "par"; the only effect would be to shift the errors to 

 some other aperture. Assuming the exposure for the//2.8 aperture to be correct, the 

 exposure time for other apertures was calculated and compared with that indicated by 

 the meters, as shown in the table. The errors were also tabulated. 



It will be observed that for the first type of meter, a very small and simple vest- 

 pocket type, the errors become cumulative because of the manner in which the progres- 



t !(,. (i. - ]^e^\l >-^i \ iftual 

 exposure meter with focusing 

 eyepiece. This meter has a 

 blue filter and viewing screen 

 covering the average field of 

 view, which enables the pho- 



