IGF^{=} AI =^{FG}AA =^{FG}IF =^{IA}GG =^{IA}FFigure 1-32.–Proportional IFGA.All image-object and focal length distancerelationship problems can be computed with thefollowing 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 sinceyou will have many uses for it in many differentapplications of photography.Study the proportional IFGA figure and note thefollowing:I - the image sizeF - the image focal distanceG - the object sizeA - the distance from lens to objectThe ratio of image size to image focal distance isthe same as the ratio of object size to object focaldistance as follows:I:F = G:AI =^{FG}AI =12 x 1030The mathematical equation resulting from thisproportion is as follows:The proportion may be written in fractional form asfollows:When solving for I:When solving for A:To clear or set apart one factor of an equation so it maybe solved, divide the equation by all factors on that sideof the equation except the one to be set apart.When solving for F:When solving for G:These four formulas are from the same equationIA = FG.Inches and feet are used in the equation thateliminates the computations required to reduce feetmeasurements to inches. However, the relation of inchesto inches and feet to feet must be maintained on therespective sides of each equation Keep I and F valuesin inches and G and A values in feet. Then, when solvingfor I or F, the result will be in inches. When solving forG or A, the result will be in feet.In the sample problems which follow, the IA = FGformula is used as though the camera were focused atinfinity.PROBLEM 1: A lens with a focal length of 12 inchesis used to photograph an object 10 feet high from adistance of 30 feet. What is the size of the image? Solvefor the unknown factor (image size) by substituting theknown factors (focal length, object size, and distance)into the equation IA = FG. The formula andcomputations are as follows:IA=FGI = 4, or image size equals 4 inchesl-29

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