Exposure - Key Terms

One of the classes I teach is "Mastering Exposure and Focus."  This class shows students how to adapt creatively to difficult lighting situations (exposure) and how to improve their skills in getting tack-sharp images (focus.)
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In this blog, I want to define some key terms relating to exposure.

Exposure.  The amount of light reaching the sensor based on four factors:  the ambient light, aperture, shutter speed, and ISO.  The ambient light is the light surrounding your subject.
Exposure Triangle.  These are the three features on a digital camera that you control:  aperture, shutter speed, and ISO.
Aperture.  The opening in the lens that is similar to the pupil in the eye.  Various aperture sizes are designated by f/stop numbers.  For example, f/4 is a wide aperture opening while f/22 is a narrow opening.  The aperture range on a typical lens looks like this:  f/4, f/5.6, f/8, f/11, f/16, f/22, f/32.  When you go from f/4 to f/5.6, you are doubling the amount of light on the sensor.  This is called a stop of light.  When you go from f/32 to f/22, you are cutting light in half.
Depth-of-Field (DOF).  This is the range of sharpness within an image.  Aperture sizes affect the DOF.  Wider apertures, such as f/4 help create a shallow DOF, which is good for close-ups and portraits.  Small apertures, such as f/16 or f/22, create a deep DOF, which is good for landscapes.  
Exposure or Shooting Mode Dial.  Advanced digital cameras offer a fully automatic shooting mode plus several creative modes:  Program, Aperture, Shutter, and Manual.  In Aperture Priority, for instance, you have creative control over DOF.  Aperture Priority is designated on your mode dial as either "A" for Nikon or "AV" for Canon.  In Shutter Priority, you have creative control over freezing or blurring action.  "S" on the mode dial for Nikon.  "TV" on the mode dial for Canon.  "TV" means time value.
Shutter Speed.  This is the amount of time your camera spends taking a picture.  It's usually measured in fractions of a second.  Advanced cameras offer a range of shutter speeds from 30 seconds to 1/4000 of a second or faster.  You can freeze most action at 1/200 of a second.  You can blur most action at 1/50 second or slower.
ISO.  This measures the sensitivity of your sensor to light.  ISO typically ranges from 100 to 6400 on most digital cameras.  Here's the range given in full stops of light:  100, 200, 400, 800, 1600, 3200, 6400.  On a sunny day, the ISO setting might be 100-200.  At twilight, however, the ISO setting might be 800-1200.  As you increase the ISO, the sensor becomes more sensitive to low light situations.  Some cameras offer Auto ISO.

When you understand the Exposure Triangle and learn to master Aperture, Shutter Speed, and ISO, you will be able to create more compelling images.