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Figure 1-23.Equal f/stops produce equal intensities
Table 1-2.Amount of light, f/stop, and Exposure Time Relationship

Photography (Basic) - Introduction to photography and other graphic techniques
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exception. All these numbers may not exactly reduce the amount of light admitted by one half, but they are sufficiently close for all practical purposes. However, all of these values are in proportion to the squares of their  numbers.  For  example,  f/4  admits  four  times  more light than f/8 because the square of f/4 is contained in the square of f/8 exactly four times. Thus, 42  =  4  x  4  =  1  6 82  =  8  x  8  =  6  4 6 4 1 6=  4 Table 1-1 shows that the amount of light admitted is  inversely  proportional  to  the  square  of  the  f/stop, while the exposure required is directly proportional to it. EXAMPLE:  The  correct  exposure  at  f/8  required  1 second. How long an exposure is required at f/16? The proportion and computation are as follows: (Old f/value)2 (Old exposure) (New f/value)2 =  (Required  exposure) 82 1 1 62=x 6 4 1 = 2 5 6 x 64x = 256 x  =  4 Thus  the  required  exposure  equals  4  seconds. Table 1-1.–Comparison of f/stops with Amount of Light to Exposure Time f/ value f/value2  squared Amount  of  light  admitted Exposure  in  seconds 4 1 6 4 1/4 4.5 (half stop) 20.25 3.2 1/3 5.6 31.36 2 1/2 8 64 1 1 11 121 1/2 2 16 256 1/4 4 22 448 1/16 8 32 1024 1/32 16 l-20







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