Figure 2-21. Effect of lens-to-subject distance

on lens-to-film distance.

c.

Use of Conjugate Distances. Let us assume that you are to photograph a

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.

I

I

6

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

distance.

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.

(3) The

last

step

is

to

use

the

formula

below

to

determine

object

distance.

Image distance

ID

18

Object Distance =

scale

or OD = S

=

OD = 36

The lens should be 36 inches from the object.