This computation can be done with lenses marked in
millimeters; however, the result will also be in
millimeters. At this point, you must convert millimeters
to inches as follows:
A = FG
I
12 inches x 12 feet
A =
9 inches
I = 305mm x 10 ft
30
= 101mm x .04 (conversion factor) = 4 inches
Where:
I = the image size
F = the focal length
G = the object size
A = the distance from the lens to the object
PROBLEM 2: A 24-inch focal-length lens is used to
photograph an object 10 feet high from a distance of 30
feet. What is the length of the image? The formula and
computations are as follows:
F =IA
G
F = 4 x 20
8
I = 8 inches
or, solving to prove the unit of measure of the result.
I = F G
A
I =24 x 10
3 0
I = 24 inches x 10 feet
30 feet
I = 8 inches or image size
I = 8 inches
As an example of a typical situation whereby you
can make use of the IA = FG formula, suppose you are
requested to make a 9-inch photograph of a board 12
feet long. This board is mounted on a wall and the
maximum distance from that wall to the opposite side
of the room is 20 feet. Is it possible to make this
photograph using an 8x10 camera equipped with a
12-inch focal-length lens?
G = IA
F
G = 9 inches x 16 feet
12 inches
The known values are object size (12-foot board),
requested image size (9 inches), and the focal length (12
inches). The unknown factor is the necessary
lens-to-subject distance required to make the
photograph using this camera. The formula and
computations are as follows:
IA = FG
A= 16 feet
The required lens-to-subject distance equals 16 feet.
The answer to this problem then would be yes, since the
required lens-to-subject distance is only 16 feet. This
allows the photographer 4 feet (20 - 16 = 4) in which to
set up and operate the camera.
PROBLEM 3: An image 4 inches long of an object 8
feet high at a distance of 20 feet is focused on the film
plane. What is the lens focal length?
IA = FG
F = 10, or focal length equals 10 inches
Another problem to illustrate the application of the
proportion I:F=G:A follows: When using an 8x10
camera equipped with a 12-inch focal-length lens to
obtain a 9-inch image from a distance of 16 feet, you
can photograph an object of what maximum length? To
solve this problem, you should have the formula and
computation as follows:
G = 12 feet
The maximum length of an object that can be
photographed with this 12-inch lens, using an image size
of 9 inches from a distance of 16 feet, is 12 feet.
Using Various Lenses
It is possible for you to take all your pictures with
only one lens. But before long, you will want to expand
your range of lenses to become a more versatile
photographer.
Within our Navy, 35mm single-lens reflex (SLR)
cameras are coming into ever-increasing use. Every
Navy photo unit should have several SLR cameras, and
l-30

Basic Photography Course












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