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.
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