EXPOSURE AND EXPOSURE DEVICES 



231 



only as indicating the relative exposures between the various apertures, as indicated 

 in the example above. 



Table VIII gives basic exposure data for the photography of outdoor subjects in 

 daylight. For purposes of simplicity, this table has been reduced to the minimum 

 amount of essential data which requires consideration and consequently should be 

 used as a guide rather than as providing exact data. It is suitable, without correction, 

 for clear days when the sun is shining unobscured and for films having speeds of Weston 

 24 to 48, and it applies for the larger part of the day. The table is supplemented with 

 footnotes outlining the conditions for which the table applies and giving data on 

 extending the range of the table. 



Table VII. — Relation between Apertures and Relative Exposure 



The exposure for outdoor subjects depends upon the quantity and quality of light 

 which is available, and these in turn depend upon the time of day and the latitude as 

 well as the state of the sky. Table IX shows the variation in light during the day for 

 clear sky with sun shining unobstructed. The table is calculated for both the northern 

 and southern hemispheres for latitudes as great as 65°. It may be used without 

 correction for fully panchromatic materials to within an hour of sunrise and sunset. 

 For orthochromatic materials, the exposures may be doubled or tripled within 2}4 hr. 

 of sunrise and sunset. The table does not show the exact exposure required for certain 

 film speeds and apertures but merely indicates the manner in which the light varies 

 throughout the day. If the correct exposure is known for a specified hour at a certain 

 latitude and a given time of the year, the table may be used to indicate the correct 

 exposure at other periods of the day and time of year, and even for other latitudes, 

 within reason. For example, suppose the correct exposure for a clear day in New 

 York City (latitude 42° N.) at 10 a.m., June (Eastern standard time or 11 a.m. day- 

 light-saving time), is found to be Ko sec. Table IX shows that the relative light for 

 this condition is 1.5. For the same film speed and aperture the exposure time may now 

 be found for some other time of day. At 5 p.m., for example, the relative exposure due 

 to decreased light is found to be 3. Thus the shutter speed should be increased by the 



