Exposure Value (EV)

by Hugh Stockton

The relation between ISO/Lens Opening (f/stop)/Shutter Speed?

It may come as a surprise, but these common terms of photography aren't just floating around un-tethered in space with only relative connections between them. In order to understand how they connect, we need to discuss another important, but seldom mentioned term:

Exposure Value (EV)

The main reason EV is not a common term is because our cameras have built in light meters that are coupled to the lens and shutter adjustments. That is convenient, but shields us from the information necessary to understand why the system works as it does. First, let's review the meanings of these terms:

1. ISO - the sensitivity of the sensor to light
   
   
2. f/stop - the size of the opening in the lens through which light must pass to reach the sensor. Beware of the reciprocal nature of this term. The f/stop 'number' "f/2.8," "f/5.6," ETC. increases as the aperture size gets smaller. A larger f/stop number allows less light to reach the sensor. "Stopping down" means to use a smaller f/stop (larger f/stop number) and decrease the amount of light. "Opening up" means to use a larger f/stop (smaller f/stop number) and increase the amount of light.
   
   
3. Shutter Speed (SS) - the length of time the light is allowed to illuminate the sensor
   
   
4. Exposure Value (EV) - the intensity of the light measured by the light meter
   
   

Note that both '2' and '3' above control the cumulative amount of light ultimately reaching the sensor. For a given f/stop and SS, the higher the EV the more light will reach the sensor. Increasing the ISO decreases the amount of light necessary for a proper exposure.

So, how does this all work to yield the proper exposure? Assume you have a dSLR set to "Average" metering and Manual Exposure control.

First, you set the ISO so the system knows how much light it needs for a proper exposure. Typically, a lower ISO will yield less noise and result in a 'cleaner' image.

Second, compose the image in the viewfinder. This will meter the scene providing an EV reading so the system knows how much light is available.

Third, set the f/stop to provide the required Depth of Field.

Fourth, set the Shutter Speed to 'center the needle' on your light meter. If you were using Aperture Priority, the camera would automatically make this adjustment for you.

OR

Step 'First'

Step 'Second'

Third, set the Shutter Speed to stop the action or provide motion blur to taste.

Fourth, set the f/stop to 'center the needle' on your light meter. If you were using Shutter Priority, the camera would automatically make this adjustment for you.

Fifth, shoot.

Sixth, view the histogram. Generally, if the histogram is mostly to the 'left' or darker side, more light (longer SS or aperture) is needed. If the histogram is mostly to the 'right' or lighter side, less light (faster SS or smaller aperture) is needed.

Seventh, return to Steps 3, 4, 5, and 6 as necessary.

Good luck.