Figure 2-21. Effect of lens-to-subject distance
on lens-to-film distance.
tool that is 12 inches long. The image is to be 6 inches long, and you are to
use a camera with a 12-inch lens to accomplish your work.
(1) The first step is to determine the scale of the photograph by using
the formula below.
Where I = Image length, 0 = Object length, and S = Scale.
SCALE = O or S = O
12 or S =
Scale is 1:2
(2) The next step is to use the formula below to determine the image
Image Distance = Focal Length x (Scale plus 1) or
ID = FL x (S + 1) or 12 x ( + 1) = 12 x 1 = 18
ID = 18
In other words, the lens should be 18 inches from the film.
Object Distance =
or OD = S
OD = 36
The lens should be 36 inches from the object.