Reciprocity And Understanding Exposure Value

In my pursuit to understand everything photographic, I recently ran across the word "reciprocity."

Merriam-Webster defines the word as "mutual dependence, action, or influence."  In photographic terms that means the relationship between shutter speed and aperture.  Change one of the values and you must also change the other to maintain proper exposure.  For example, lets assume a proper exposure with ISO 100, shutter of 1/125 and an aperture setting of f8.  If you need more depth of field, you'll have to stop down your aperture.

For an example, let's assume we're taking an exposure of a landscape and we need a very deep depth of field.  Let's stop down the aperture setting and move to f16.  In order to stay in the same EV zone, we must adjust our shutter speed to compensate for the loss of light.  Since we lost half of our light, we must allow roughly twice as much time to obtain the same exposure. So our new shutter speed will be 1/30. 

A second example would be taking a portrait.  We want a nice buttery smooth bokeh behind our subject.  So we will open up the aperture to a setting of f4.  We've now double the amount of light being let into our sensor.  Now have need to reduce the time of exposure by half, or 1/500.  This leaves us still in the same EV zone and having a correctly exposed image.

The math seems off, but there are many examples of charts on the web to use as reference. 

The understanding of reciprocity and usage of an EV chart can help a photographer properly expose and determine the appropriate depth of field needed to capture a desired scene.

In my own personal opinion,  I feel that the "program" mode on most new digital SLR's use the EV chart to gain correct exposure.  I think there may have to be some testing on that in the future.


On a side note, EV should not be confused with Ansel Adams' "zone system."  Although similar sounding, Mr Adams' system uses 11 different zones based on tonal values and he adjust exposure according to which part of the scene has the most impact.

This post a little long and somewhat "dry."  I apologize if it bores you, I just really think it's a basic concept that gets forgotten about.

Here is a portrait taken during an "un-wppi" party.