I have to disagree a bit. In essence, basing your exposure on important subject matter is exactly right. The problem with averaging several important areas of subject matter, especially with an unforgiving medium like slide film, is that you may end up with none of them being exposed as you like. Yes, averaging works in many cases, but in cases of high contrast, we need to make a better choice.
With black-and-white film, one "pegs" to the shadows. One develops and/or prints to get desired contrast.
For color negative films, the same thing, but with more limitations, applies.
For slide films and B&W reversal films, pegging to the highlights is a good practice for non-portrait scenes. In contrasty situation, the shadows, and to a lesser extent, the lower midtones, may suffer. For portraits, basing exposure on a skin tone is good. In contrasty portraits, then, the highlights and shadows would suffer. The portrait may not be acceptable; softer lighting may be the only solution.
Average meters work well in many situations and are pretty good with films that have lots of exposure latitude. They tend to have a higher failure rate with less-forgiving materials and contrasty scenes. There isn't always "an exposure that protects both the high and low tones." Sometimes you lose them both and end up with the mid-tone somewhere you don't like.
You can only place one value; others fall where they may. It's good to know this and use your meter help you make an informed decision.
So, to the OP I would advise the following: Identify your most important tonality. Evaluate the contrast of the scene. Mentally place your important value and then imagine what will happen with the others. Slide film has about a five-stop range. If you place a highlight, you can see which values will go black easily. If you place a mid-tone, you can imagine what will happen with shadows and highlights equally easily.
For low-contrast scenes, averaging often works really well. I'd use it in many cases. For scenes of higher contrast, I spend a good bit of time deciding where to place what. For landscapes with white water, it's the water: meter it and overexpose two stops. The shadows go black if the scene is too contrasty. There's nothing to do about that. Often, the shot is still acceptable. For a portrait, the skin tones and maybe let the highlights go blank white.
Sometimes the contrast of a scene precludes making a good photograph on slide film. In that case, don't shoot and save yourself the trouble and money.
There are some things you can do to tame the contrast of slide film: pre-exposure often helps, so does fill-flash and reflectors for portraits. For landscapes, waiting for a cloud to partially cover the sun is often gratifying.
Hope this helps,
Last edited by Doremus Scudder; 05-01-2014 at 07:30 AM. Click to view previous post history.