EXPOSURE AND EXPOSURE DEVICES 223 



specifying film speed, the larger numerics indicate the faster (more sensitive) materials, 

 i.e., those requiring less exposure to produce a given density on the negative. All 

 other factors remaining constant, the quantity of light reaching the film may be 

 reduced more and more, for a given density, the higher the speed of the film, provided 

 the speed numbers are all in the same system of measurement. The speed numbers 

 in the various systems do not progress in the same proportion as film speed increases, 

 however. 



So far as concerns the determination of exposure, the film-speed number has its 

 principal value in connection with exposure meters which are calibrated in one of the 

 various systems. It may be presumed, therefore, that, when a film and an exposure 

 meter are both marked in the same system of film-speed units, the exposure conditions 

 calibrated on the exposure meter will apply directly for the film marked in that system. 

 Frequently it happens that an exposure meter is calibrated for one system of speed 

 rating, whereas the film speed or sensitivity is given in another system. By means of 

 the film-speed conversion table (see Appendix B), it is possible to determine approxi- 

 mately the equivalent speed of films in the various systems and, from the appropriate 

 conversions, to make use of the dissimilar ratings of the film and meter calibration. 

 In making such conversions, however, it must be remembered that the various speed 

 systems may have been based on fundamentally different methods of measurement 

 and that the equivalent conversions are determined empirically from measurements 

 on a large number of films. Thus, while the conversion factors given represent aver- 

 age values, it is quite possible that some particular emulsion may not fit average 

 conditions, and in such instances the conversion equivalents may be considerably in 

 error. 



The spectral sensitivity of the film has an important bearing on the exposure. 

 Panchromatic materials are sensitive to all visible light but have maximum sensitivity 

 to blue light and decreasing sensitivity throughout the rest of the visible spectrum. 

 However, the sensitivity even to red light is sufficient that no difficulty will be encoun- 

 tered when objects of various colors are photographed or when lights of various colors 

 are used for purposes of illumination. Orthochromatic materials, which are insensi- 

 tive to the red and possibly orange portions of the visible spectrum, require longer 

 exposures in yellow or orange light than panchromatic materials having the same 

 daylight sensitivity. Ordinary or orthonon materials are sensitive only to the blue 

 region of the visible spectrum, so that it may be quite impossible to make photographs 

 with red or orange light, no matter how long an exposure is made. It is because of 

 the change in spectral sensitivity that most films are now given two film speed ratings. 

 One of these is a rating for light of daylight quality, having approximately equal energy 

 in all portions of the visible spectrum. The other film speed is given for illumination 

 by tungsten lamps operated at temperatures at about 2800 to 3000°K. For photo- 

 flash and photoflood illumination, in which the lamps operate at temperatures of 

 3200 to 3300°K., an appropriate film-speed rating will usually be somewhere between 

 the dajdight and the tungsten film-speed rating, although, if there is any doubt, the 

 rating for tungsten lamps should be used. 



If we know as reference conditions the aperture stop /o, the shutter speed to, the 

 film speed So, and the intensity of illumination /o for certain exposure conditions, we 

 may determine new values of aperture stop /, shutter speed t, film speed, and intensity 

 of illumination / for a properly exposed negative. From Eq. (18) we have 



* - ^"^G-.)'(i)(T)(Y)(^^ + "' (=«» 



As an example, suppose an aperture stop of //8 is required for certain conditions 

 when the film speed has a rating of Weston 8 and the shutter speed is J^o sec. If the 



