40 PHOTOGRAPHIC FACTS AND FORMULAS 
lens on the same object, the lens being placed with the back 
combination to the front. Mark the position as before, then 
refocus after removing the front combination, really the back 
one now in front. The distance d’ between the two positions 
is f’F —f. Hence dd’ = F’, that is to say, by multiplying 
the two distances d and d’ together and extracting the square 
root, we get the focal length. 
 Lockett’s Method—A. Lockett suggested the following 
method: First draw two short vertical lines at about the 
center of the focussing screen, exactly 1 inch apart and 
parallel with each other. Focus sharply on a far distant 
object, such as a remote church spire or factory chimney, 
and mark carefully on the camera baseboard the exact posi- 
tion of any convenient part of the moving lens front. This 
may be called the infinity mark. Now measure off 1 inch in 
advance of this mark, and rack out the camera until the same 
point of the front is against this l-inch mark. Fix up a 
foot rule at about the height of the lens, and move the whole 
camera to and fro, without any other adjustment, until the 
rule is in the sharpest possible focus at full aperture, and 
with the commencement or zero of the graduations coinciding 
with one of the pencil lines. Then the number of inches of 
the rule seen on the ground glass between the two pencil lines 
will be equal to the focal length of the lens. This method is 
based on the following reasoning: let F be the equivalent 
focus, and r the ratio or proportion between the size of the 
image and the object. Then the minor conjugate focus, or 
the distance from lens to ground glass, is F + F~r. When 
the camera front is set 1 inch from the infinity mark, then 
F —~r equals 1 inch and must also be equal to F ~ F; there- 
fore r equals F. It is not essential to adhere to the 1-inch 
extension, and a greater distance will eliminate errors in 
