PROBLEM: Determine the distance that is requiredbetween 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 (4:16or 5:20) may be reduced by using the following formula:R =^{4}116=4R =^{8}_{4}= 2Likewise:R =^{5}120^{=}4Substituting the figures into the formula:Object focal distance = F + (F R)= 10+(10+ 1/4)= 50 inchesImage focal distance = F + (F x R)= 10 + (10 x 1/4)= 12.5 inchesTherefore, the camera lens must be 50 inches from theprint and the film must be 12.5 inches from the lens tomake a 4x5-inch image of a 16x20 print using a 10-inchlens.EXAMPLE 2: Make a full-length portrait of a man 6feet (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 tomake this photograph?The ratio is 5:72, which reduces toR =^{5}172^{=}14.4Substituting the formula:Image focal distance = 10 + (10 x^{1})= 10.7 inches14.4Object focal distance = 10 + (10 +^{1}) = 154 inches14.4Adding 10.7 inches and 154 inches and converting tofeet gives a film to subject distance of 13.7 feet.However, there must be enough space added to thisdistance to allow a background behind the subject andoperating space behind the camera. Three or four feet ateach end is about the minimum for good work Thus, ifthe room is not at least 20 feet long (13.7 + 6 = 19.7), aportrait this size cannot be made with a 10-inch lens.EXAMPLE 3: A diagram 4 inches square is to bephotographed so the image on the film is 8 inchessquare. Using a 10-inch lens, how much bellowsextension, or camera length, is required? The ratio hereis 8:4, orThe image focal distance equals the bellows extensionor the required length of the camera.Substituting:Image focal distance = 10 + (10 x 2)= 30 inchesIf the camera does not have sufficient bellows extensionto allow the film to be placed 30 inches from the lens,the required negative or image size cannot be made withthis camera and lens.It is not difficult to calculate the various distancesfor different jobs. The photographer also saves the timeand unnecessary work usually required by thetrial-and-error method.Image/Object RelationshipThe size of the image formed by a lens is dependentupon the following:The size of the subjectThe lens-to-subject distanceThe lens focal lengthThe size of the image of any object at a given distanceis directly proportional to the focal length of the lensbeing used. That is, when a given object at a givendistance appears 1 inch high on the focal plane when a3-inch lens is used, it appears 2 inches high when a6-inch lens is used and l/2 inch high when a 1 1/2-inchlens is used.The proportion illustrated in the following figure isthe basis of the equation commonly used for solvingimage-object and focal length distance relationshipproblems (fig. 1-32).1-28