The Law of Reciprocity in Photography: Part I

For over a year now I have been wanting to teach a photography workshop. This is finally happening next month.  In preparation for teaching the workshop I will be brainstorming on what information to cram into a 2 hour workshop. So, here we go:

RECIPROCITY LAW: Principal according to which the density of the image formed when an emulsion is developed is directly proportional to the duration of the exposure and the intensity of the light. (John Hedgecoe's Complete Photography Course)

Ok, so that probably doesn't make any sense in the modern digital era.  My understanding of the LAW of RECIPROCITY as it relates to photography is well stated here:

"Thus, once you have the correct level of light for perfect exposure you can choose to increase aperture by one stop and trade this off with a doubling of the shutter speed (halving of the shutter duration). Twice and much light coming in for half the time = same amount of light.

This allows the photographer to retain the same exposure but change either aperture or shutter for artistic or practical reasons.

In addition to this the photographer may wish to over expose or under expose. Understanding the law of reciprocity allows them to do this in a controlled and intuitive manner, understanding the exposure differences on the image PLUS the different changes to the depth of field or any motion blur increase or decrease." Trigwell, Keith. "Understanding Reciprocity in Photography." Web log post. Picture Correct. N.p., n.d. Web. <>.