what's the distance needed between a 35mm camera lens and a face for face to be 1&3/8inch long on a 4X6 print

Question

BAsically what is the math behind the real size of an object, the distance to the lens, the focal length(35mm etc) and the size of the object when printed?

Answer 1

Very simply, object height/object distance = image height/image distance. Assuming size of face = 9", 35mm lens and 35mm film enlarged 4x:

image size on film = 1.375/4 = 0.344"
focal length in inches = 35/25.4 = 1.378

9/d = 0.344/1.378
d = 9x35/.344x25.4 = 36 inches