PROBLEM:  Determine  the  distance  that  is  required between the film and the lens (the image focal distance) and the necessary distance between the lens and the print (the object focal distance). The ratio between the film size and the print size (4:16 or 5:20) may be reduced by using the following formula: R  =⁴ 1 1 6 = 4 R   =⁸₄  =   2 Likewise: R   =⁵ 1 2 0⁼ 4 Substituting  the  figures  into  the  formula: Object focal distance = F + (F R) =  10+(10+  1/4) = 50 inches Image focal distance = F + (F x R) = 10 + (10 x 1/4) = 12.5 inches Therefore, the camera lens must be 50 inches from the print and the film must be 12.5 inches from the lens to make a 4x5-inch image of a 16x20 print using a 10-inch lens. EXAMPLE 2: Make a full-length portrait of a man 6 feet (72 inches) tall using a 10-inch focal-length lens, and make the image on the film 5 inches long. PROBLEM: How much studio space is required to make  this  photograph? The ratio is 5:72, which reduces to R  =⁵ 1 7 2 ⁼14.4 Substituting  the  formula: Image focal distance = 10 + (10 x¹    )= 10.7 inches 14.4 Object focal distance = 10 + (10 +¹   ) = 154 inches 14.4 Adding 10.7 inches and 154 inches and converting to feet gives a film to subject distance of 13.7 feet. However, there must be enough space added to this distance to allow a background behind the subject and operating space behind the camera. Three or four feet at each end is about the minimum for good work Thus, if the room is not at least 20 feet long (13.7 + 6 = 19.7), a portrait this size cannot be made with a 10-inch lens. EXAMPLE 3: A diagram 4 inches square is to be photographed so the image on the film is 8 inches square.  Using  a  10-inch  lens,  how  much  bellows extension,  or  camera  length,  is  required?  The  ratio  here is 8:4, or The image focal distance equals the bellows extension or the required length of the camera. Substituting: Image focal distance = 10 + (10 x 2) = 30 inches If the camera does not have sufficient bellows extension to allow the film to be placed 30 inches from the lens, the required negative or image size cannot be made with this  camera  and  lens. It is not difficult to calculate the various distances for different jobs. The photographer also saves the time and   unnecessary   work   usually   required   by   the trial-and-error   method. Image/Object   Relationship The size of the image formed by a lens is dependent upon  the  following: The size of the subject The  lens-to-subject  distance The lens focal length The size of the image of any object at a given distance is directly proportional to the focal length of the lens being used. That is, when a given object at a given distance appears 1 inch high on the focal plane when a 3-inch lens is used, it appears 2 inches high when a 6-inch lens is used and l/2 inch high when a 1 1/2-inch lens is used. The  proportion  illustrated  in  the  following  figure  is the basis of the equation commonly used for solving image-object  and  focal  length  distance  relationship problems  (fig.  1-32). 1-28