There are three primary aspects to a photographic exposure:

  1. How sensitive is the recording material?
  2. How bright is the light contacting the recording material?
  3. How long is the recording material exposed to the light?

I’ve found it useful to map the task of getting the “right” exposure to the act of filling a bucket (the film or digital sensor) with water (light) through a hose (the camera and lens).

  1. The sensitivity of the recording material (measured in ISO) is akin to the size of the bucket.  A smaller bucket is analogous to a more sensitive recording material (higher ISO).
  2. The lens's aperture (f/number) maps to the flow rate of the hose.  A bigger aperture (smaller f/number) is like using a fatter hose to fill the bucket.
  3. The shutter speed (exposure time) is the duration the hose is turned on.

Your job as photographer is to fill the bucket perfectly.

If you make a try filling the bucket and it is not quite full, this is analogous to an underexposure, or dark exposure.  To do it better, you need more water.  So either you need a higher flow rate hose (wider aperture) or you need to keep the hose running longer (longer shutter speed).  

If the bucket overflows, you either need to constrict the hose you are using (smaller aperture) or turn off the hose sooner (shorter shutter speed).

Makes sense?

Here are some additional explanations of the three axes of an exposure:

ISO (Sensitivity)

Film or digital sensor sensitivity is measured in ISO units.  The higher the ISO number, the more sensitive the recording material is.  So ISO 200 is twice as sensitive as ISO 100, and thus requires half the light to get an equivalent exposure.

Aperture

A lens’s job is to collect and focus light.  A larger diameter lens is able to collect more light than a smaller diameter lens.  Therefore, a larger diameter lens can project a brighter picture onto the recording material.  It’s not always desirable to have the lens gathering as much light as it possibly can, so most camera lenses have an adjustable diaphragm inside to restrict the amount of light that passes through the lens.  It’s very similar to having a really big window in a room with shades over it that you can close.  Sometimes it’s great to let the light stream in, but not always.

Aperture is expressed as f/number which is the focal length of the lens divided by the diameter of the lens opening.  So a 100mm lens that had a 25mm opening would have an aperture of f/4 (100 / 25 = 4).  The same lens with a 50mm opening would have an aperture of f/2 (100 / 50 = 2).

Doubling the area, not the diameter, of the lens opening doubles the amount of light striking the recording material.  So in the example above, if we look at circle areas, the f/4 aperture has an area of 490 mm^2 ((( 25mm / 2 ) ^ 2 ) * pi) while the f/2 aperture has an area of 1960mm^2.  The f/2 aperture lets in four times the light as the f/4 aperture.

Shutter Speed

This is the duration the recording material is exposed to light.  It it typically represented as a number like “1000” or “60”.  In these cases, this is the reciprocal of the actual duration of exposure.  When a camera shows an exposure of “1000” it really is 1/1000th of a second.

An exposure of 1/30 second lets in twice as much light as 1/60 second.

Now get out there and shoot, shoot, shoot.