PROBLEM: Determine the distance that is required
between the film and the lens (the image focal distance)
and the necessary distance between the lens and the print
(the object focal distance).
The ratio between the film size and the print size (4:16
or 5:20) may be reduced by using the following formula:
R =84 = 2
Substituting the figures into the formula:
Object focal distance = F + (F
= 10+(10+ 1/4)
= 50 inches
Image focal distance = F + (F x R)
= 10 + (10 x 1/4)
= 12.5 inches
Therefore, the camera lens must be 50 inches from the
print and the film must be 12.5 inches from the lens to
make a 4x5-inch image of a 16x20 print using a 10-inch
EXAMPLE 2: Make a full-length portrait of a man 6
feet (72 inches) tall using a 10-inch focal-length lens,
and make the image on the film 5 inches long.
PROBLEM: How much studio space is required to
make this photograph?
The ratio is 5:72, which reduces to
Substituting the formula:
Image focal distance = 10 + (10 x1 )= 10.7 inches
Object focal distance = 10 + (10 +1 ) = 154 inches
Adding 10.7 inches and 154 inches and converting to
feet gives a film to subject distance of 13.7 feet.
However, there must be enough space added to this
distance to allow a background behind the subject and
operating space behind the camera. Three or four feet at
each end is about the minimum for good work Thus, if
the room is not at least 20 feet long (13.7 + 6 = 19.7), a
portrait this size cannot be made with a 10-inch lens.
EXAMPLE 3: A diagram 4 inches square is to be
photographed so the image on the film is 8 inches
square. Using a 10-inch lens, how much bellows
extension, or camera length, is required? The ratio here
is 8:4, or
The image focal distance equals the bellows extension
or the required length of the camera.
Image focal distance = 10 + (10 x 2)
= 30 inches
If the camera does not have sufficient bellows extension
to allow the film to be placed 30 inches from the lens,
the required negative or image size cannot be made with
this camera and lens.
It is not difficult to calculate the various distances
for different jobs. The photographer also saves the time
and unnecessary work usually required by the
The size of the image formed by a lens is dependent
upon the following:
The size of the subject
The lens-to-subject distance
The lens focal length
The size of the image of any object at a given distance
is directly proportional to the focal length of the lens
being used. That is, when a given object at a given
distance appears 1 inch high on the focal plane when a
3-inch lens is used, it appears 2 inches high when a
6-inch lens is used and l/2 inch high when a 1 1/2-inch
lens is used.
The proportion illustrated in the following figure is
the basis of the equation commonly used for solving
image-object and focal length distance relationship
problems (fig. 1-32).