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Image/Object Relationship
Using Various Lenses

Photography (Basic) - Introduction to photography and other graphic techniques
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I G F=   A I  =FG A A  =FG I F    =IA G G    =IA F Figure 1-32.–Proportional IFGA. All   image-object   and   focal   length   distance relationship  problems  can  be  computed  with  the following  simple  proportion: The image size (I) is to the image focal distance (F) as the object size (G) is to the object focal distance (A) You should thoroughly understand this equation since you will have many uses for it in many different applications  of  photography. Study the proportional IFGA figure and note the following: I - the image size F - the image focal distance G - the object size A - the distance from lens to object The ratio of image size to image focal distance is the same as the ratio of object size to object focal distance  as  follows: I:F = G:A I     =FG A I     = 12 x 10 3 0 The mathematical equation resulting from this proportion  is  as  follows: The proportion may be written in fractional form as follows: When solving for I: When solving for A: To clear or set apart one factor of an equation so it may be solved, divide the equation by all factors on that side of the equation except the one to be set apart. When solving for F: When solving for G: These  four  formulas  are  from  the  same  equation IA = FG. Inches  and  feet  are  used  in  the  equation  that eliminates  the  computations  required  to  reduce  feet measurements to inches. However, the relation of inches to inches and feet to feet must be maintained on the respective sides of each equation Keep I and F values in inches and G and A values in feet. Then, when solving for I or F, the result will be in inches. When solving for G or A, the result will be in feet. In the sample problems which follow, the IA = FG formula is used as though the camera were focused at infinity. PROBLEM 1: A lens with a focal length of 12 inches is used to photograph an object 10 feet high from a distance of 30 feet. What is the size of the image? Solve for  the  unknown  factor  (image  size)  by  substituting  the known factors (focal length, object size, and distance) into   the   equation   IA   =   FG.   The   formula   and computations  are  as  follows: IA=FG I = 4, or image size equals 4 inches l-29







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