Understanding Exposure - The Sunburn Analogy

The creation of a photographic image as the result of exposure to light is the result of a "photo-chemical" reaction. To make this easier to understand, I might suggest we think of another photo-chemical reaction: a sunburn. A sunburn happens when your skin gets too much exposure to sunlight. Exposure can be better understood if we consider what conditions come about to make a sunburn happen. For instance, you might get a sunburn if you:

A) have fair skin
B) are in bright sun
C) stay out in the sunlight for 2 hours

These three variables are present in any photochemical reaction:

A) Sensitivity = How readily does the chemical react to light and how fast the reaction takes place
B) Amount = How much light is the chemical exposed to
C) Duration = How long is the chemical exposed to light

For instance, you may also be sunburned if:

A) Have medium skin
B) Are in bright sun
C) stay in the sun for 4 hours

In this above case, the additional exposure time may be required to compensate for the medium skin that doesn't react has quickly to the light

Or:

A) Have fair skin
B) Are in cloudy light
C) stay in the sun for 8 hours

In this case, the additional exposure time compensates for the indirect sunlight

Or:

A) Have medium skin
B) Are in cloudy light
C) Stay in the sun for 20 hours

This last example may not actually result in a sunburn, but the concept is what's important. IN your cameras, the same conditions can be manipulated through settings:

A) The "ISO" controls how sensitive the camera is to light and increasing it can help you shoot more effectively in low light conditions
B) The "Aperture" controls how much light enters the camera
C) The Shutter controls the duration of exposure to light or the "exposure time"