This computation can be done with lenses marked in millimeters;  however,  the  result  will  also  be  in millimeters.  At  this  point,  you  must  convert  millimeters to  inches  as  follows: A   = FG I 12 inches x 12 feet A   = 9  inches I    = 305mm  x  10  ft 30 = 101mm x .04 (conversion factor) = 4 inches Where: I = the image size F = the focal length G = the object size A   = the distance from the lens to the object PROBLEM 2: A 24-inch focal-length lens is used to photograph an object 10 feet high from a distance of 30 feet. What is the length of the image? The formula and computations are as follows: F     = IA G F   = 4  x  20 8 I = 8 inches or, solving to prove the unit of measure of the result. I    = FG A I   = 24   x   10 3 0 I   = 24 inches x 10 feet 30  feet I = 8 inches or image size I = 8 inches As an example of a typical situation whereby you can make use of the IA = FG formula, suppose you are requested to make a 9-inch photograph of a board 12 feet long. This board is mounted on a wall and the maximum distance from that wall to the opposite side of the room is 20 feet. Is it possible to make this photograph using an 8x10 camera equipped with a 12-inch  focal-length  lens? G  = IA F G  = 9 inches x 16 feet 12 inches The known values are object size (12-foot board), requested image size (9 inches), and the focal length (12 inches).   The   unknown   factor   is   the   necessary lens-to-subject   distance   required   to   make   the photograph  using  this  camera.  The  formula  and computations  are  as  follows: IA = FG A=  16  feet The required lens-to-subject distance equals 16 feet. The answer to this problem then would be yes, since the required lens-to-subject distance is only 16 feet. This allows the photographer 4 feet (20 - 16 = 4) in which to set up and operate the camera. PROBLEM 3:   An image 4 inches long of an object 8 feet high at a distance of 20 feet is focused on the film plane. What is the lens focal length? IA = FG F = 10, or focal length equals 10 inches Another  problem  to  illustrate  the  application  of  the proportion I:F=G:A follows: When using an 8x10 camera equipped with a 12-inch focal-length lens to obtain a 9-inch image from a distance of 16 feet, you can photograph an object of what maximum length? To solve this problem, you should have the formula and computation  as  follows: G  =   12  feet The maximum length of an object that can be photographed with this 12-inch lens, using an image size of 9 inches from a distance of 16 feet, is 12 feet. Using Various Lenses It is possible for you to take all your pictures with only one lens. But before long, you will want to expand your  range  of  lenses  to  become  a  more  versatile photographer. Within our Navy, 35mm single-lens reflex (SLR) cameras are coming into ever-increasing use. Every Navy photo unit should have several SLR cameras, and l-30