When not doing flash photography, there are four things that determine whether a scene will be properly exposed. They are:

  1. The amount of light falling on the scene (we'll call this Exposure Value (EV)).
  2. The sensitivity (ISO) of the film.
  3. The aperture (f/stop).
  4. The shutter speed.

A bright sunny day has an EV of 15. The "Sunny f/16" rule says that correct exposure for this scene at f/16 is the closest reciprocal of the film speed. So for ISO 100 film, the correct exposure is 1/125 second at f/16. For ISO 200 film, the correct exposure is 1/250 second at f/16. For ISO 400 film, a correct exposure is 1/500 at f/16. For ISO 800 or 1000 film, 1/1000 second at f/16. Different scenes have different EVs. Open shade has an EV of 12. Sunsets are at EV 11. Brightly lit home interiors are at EV 6, average home interiors are at EV 5, nighttime scenes away from city lights and under a full moon are at EV -3.

There are many equivalent exposure values. Let's suppose that you've got ISO 400 film loaded in your camera, and your scene is an EV 15 scene. The following aperture/shutter speed combinations will give you correct, equivalent exposures (note that not all of these combinations may be available on your camera (for example, my camera's top shutter speed is 1/2000 second so the 1/4000 and 1/8000 second exposures are not available to me without filters, which is outside the scope of this post)):

  • 1/250 second at f/22
  • 1/500 second at f/16
  • 1/1000 second at f/11
  • 1/2000 second at f/8
  • 1/4000 second at f/5.6
  • 1/8000 second at f/4

For a good write-up about exposure when you don't have a light meter handy, along with two handy charts to calculate f/stop and shutter speed combinations for a given scene, Fred Parker has a good write-up at http://www.fredparker.com/ultexp1.htm.

Flash photography is a different beast. Every flash has a guide number. My camera's built-in flash's guide number is 42 at ISO 100. Consult your camera and/or flash manual for your flash's guide number. Guide numbers vary with film speed. With flash photography, shutter speed is less of an issue as long as it is no faster than your flash sync speed (consult your camera's manual to find the flash sync speed). For straight flash photography, divide your flash's guide number at the ISO you are using (remember, guide numbers vary with film speed) by the camera-to-subject distance. This will give you the f/stop to use to take the photo. For example, if my camera is loaded with ISO 100 film, and my flash's guide number is 42, and my camera-to-subject distance is 10 feet, then 42/10=4.2. I can set my aperture to f/4 and my subject will be properly exposed.

Note that guide numbers can be given in feet or meters. My camera's manual actually gives me the guide numbers in meters but I'm used to working in feet. Multiply meters by 3.28 to get feet, or divide feet by 3.28 to get meters.

I hope this helps without confusing you.
ME Super