Subject -- Light Source Distance Change for Desired f/stop

Occasionally you need to know where move the subject or light source to be able to make the light falling on the subject correspond to a particular f/stop.  For example, you might need to know the distance to place the fill light to achieve a desired light ratio or you might need to shoot at a particular f/stop for depth of field purposes.  The calculator below is based on the law of inverse squares which states that the intensity of illumination is inversely proportional to the square of the distance between the source and subject.

Assume for example that the subject is presently 10 feet from a window and your f/stop is f/8.  If you would like for the exposure to be f/11 enter 10 for the original distance and 1 for the f/stop change and the new subject source distance would calculate to be about 7 feet. 










Change in Subject-Source Distance 

for Desired f/stop



Original Distance

F/stop Change*

New Distance





















*For a larger aperture enter a negative number. 
For example, if you are now at f/8 and want to use f/5.6 enter -1.
















All Text and Images © Joe Miller, 2004