This computation can be done with lenses marked inmillimeters; however, the result will also be inmillimeters. At this point, you must convert millimetersto inches as follows:A =FGI12 inches x 12 feetA =9 inchesI =305mmx10ft30= 101mm x .04 (conversion factor) = 4 inchesWhere:I = the image sizeF = the focal lengthG = the object sizeA = the distance from the lens to the objectPROBLEM 2: A 24-inch focal-length lens is used tophotograph an object 10 feet high from a distance of 30feet. What is the length of the image? The formula andcomputations are as follows:F =IAGF =4x208I = 8 inchesor, solving to prove the unit of measure of the result.I =FGAI =24 x 1030I =24 inches x 10 feet30 feetI = 8 inches or image sizeI = 8 inchesAs an example of a typical situation whereby youcan make use of the IA = FG formula, suppose you arerequested to make a 9-inch photograph of a board 12feet long. This board is mounted on a wall and themaximum distance from that wall to the opposite sideof the room is 20 feet. Is it possible to make thisphotograph using an 8x10 camera equipped with a12-inch focal-length lens?G =IAFG =9 inches x 16 feet12 inchesThe known values are object size (12-foot board),requested image size (9 inches), and the focal length (12inches). The unknown factor is the necessarylens-to-subject distance required to make thephotograph using this camera. The formula andcomputations are as follows:IA = FGA= 16 feetThe required lens-to-subject distance equals 16 feet.The answer to this problem then would be yes, since therequired lens-to-subject distance is only 16 feet. Thisallows the photographer 4 feet (20 - 16 = 4) in which toset up and operate the camera.PROBLEM 3: An image 4 inches long of an object 8feet high at a distance of 20 feet is focused on the filmplane. What is the lens focal length?IA = FGF = 10, or focal length equals 10 inchesAnother problem to illustrate the application of theproportion I:F=G:A follows: When using an 8x10camera equipped with a 12-inch focal-length lens toobtain a 9-inch image from a distance of 16 feet, youcan photograph an object of what maximum length? Tosolve this problem, you should have the formula andcomputation as follows:G = 12 feetThe maximum length of an object that can bephotographed with this 12-inch lens, using an image sizeof 9 inches from a distance of 16 feet, is 12 feet.Using Various LensesIt is possible for you to take all your pictures withonly one lens. But before long, you will want to expandyour range of lenses to become a more versatilephotographer.Within our Navy, 35mm single-lens reflex (SLR)cameras are coming into ever-increasing use. EveryNavy photo unit should have several SLR cameras, andl-30

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