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A Light on Physics

F-Number and Exposure Time


Richard T. Brundage
Department of Physics, Astronomy, and Engineering
St. Cloud State University


This case study presents students with the challenge of working together in teams to win a photography contest. Some team members believe in aperture priority: pictures should be taken with as large an f-number as possible. The other members advocate exposure time priority: pictures should be taken with as short an exposure time as possible. Each of these positions has its advantages and disadvantages, which the advocates need to research before coming to class, and then in class come to consensus as to a single pair of settings for f-number and exposure time to use in order to create the most artistic photograph. The case uses optics to illustrate methods and applications of science to non-science majors.


  • To understand the competing demands for light and depth of field in photography.
  • To understand the reciprocal nature of exposure time and f-number by showing that different pairs of settings can lead to proper exposure of the film.


Optics; aperture; camera; focal length; f/#; f-number; exposure time; photography; lens; light; depth of field; SLR

Topical Areas


Educational Level

Undergraduate lower division



Type / Methods

Dilemma/Decision, Role-Play



Subject Headings

Physics  |   Science (General)  |  

Date Posted


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