Meters give you the proper exposure for middle-tone grey, no matter what you point them at. So if you point them at the darkest part of a scene, they give you a setting that if you chose it, would give you medium grey instead of black. If you point the meter at the lightest part of a scene, that would do the same.
To keep this simple, let's assume Kodak Gold ISO 200 film and a preferred aperture of f8 - a nice outdoor scene.
You meter the darkest part of the scene - 1/30 @ f8.
You meter the lightest part of the scene - 1/1000 @ f8.
There are how many stops of exposure between 1/30 and 1/1000 in this equation?
1/30, 1/60, 1/125, 1/250, 1/500, 1/1000. There are 6 stops of latitude between the darkest point and the lightest point in your scene.
Now, you should also know the approximate latitude of your film. Kodak says that Gold 200 will capture up to 2 stops underexposure and 3 stops overexposure. That's six stops of latitude.
Therefore, you could set your camera to f8 and 1/125 or f8 and 1/250 and capture all of the light values present in the scene. No shadows should be blocked up and detail lost - no highlights should be blown out white with details similarly lost.
Now, you might have just put your meter on non-spot mode and metered the whole scene at one time - or spot metered the point you feel strongly is 'medium tone' in the scene - or used your camera's TTL meter, and you might very well have gotten the same result, and a lot faster, too.
So why bother?
Well, sometimes the difference in light values between the darkest spot in your scene and the lightest spot is more than your film can handle. Something is going to get tossed. If you shoot color slide film, you have a lot less range to work with - more like 2 or 3 stops range total. With digital, something inbetween.
So, the more you understand what the 'real' edges of your exposure are, the more you are able to INTENTIONALLY PLACE your exposure just where you want it. It won't stop you from losing shadow or blowing out highlights when the scene just contains too many stops of light value between dark and light - but it allows you to choose where you want to place it. Perhaps in a given scene, you don't mind losing the detail in the shadows to black - but you want to avoid blowing out the sky. Whatever you decide.
The other alternative is to bracket. Which also works, but of course, if you bracket by large amounts, you could end up with one slightly too hot and another slightly too cold, and nothing that's 'just right'.
I should also say that some advanced meters will do the math for you. You meter the bright and the dark spots, and it figures out what you should shoot at for an average value. You still have to decide if you want to adjust up or down for the result you want, though - it does not know if you have decided to sacrifice dark or light for any given scene where the light range exceeds the film capacity.
Hope this helps!
Best Regards,
Bill Mattocks
