All metering strategies have one common goal: to expose the film enough, but not too much.
Enough = desired detail and separation in the shadows. Too much = grossly overexposing and blocking the highlights (by this I mean ending up with highlight values on the shoulder of the film where they have no separation) or, especially with smaller films, getting a denser-than-needed negative that ends up showing more grain than desired.
There's a large window of "latitude" between the two extremes. And, there are many ways to hit the sweet spot.
Key to all of them is being smarter than your meter. The simpler the meter, the smarter you have to be and the more know-how you have to use when interpreting readings.
Fancy-dancy in-camera meters these days with matrix metering, etc., etc. are fairly foolproof. If your negatives are consistently underexposed, use exposure compensation to rate your film at a different E.I. Otherwise, you only really need be aware of really contrasty situations (see below).
Averaging meters meters give you a mean-value reading of the light they see. Learning to point them at an area that has an "average" amount of luminance is key here. Pointing one up at the bright sky will end up underexposing shadows, for example. And, since the meter just gives you a middle value, scenes with bright light sources or lots of contrast (wide range of luminances) will tend to underexpose the shadows. You need to recognize these situations and compensate by underexposing. And you need to recognize situations that are predominantly dark or light (the black-cat-in-the-coalbin vs the polar-bear-in-the-snow situations) and compensate for these by under- and overexposing respectively. Practice and experience are key here.
Incident metering is similar, except you measure the light falling on the subject, not the subject itself. This works splendidly for average situations, but not for when there is a large difference between lit and shaded portions of the scene. In these cases, it is really best to take a reading from the main light source and another from the shadow. Interpreting these two readings well allows one to find the optimum exposure in scenes with extreme lighting ratios. I don't use incident metering, so I'll defer to those who know better.
Spot metering allows one to (more-or-less) read individual values in the scene. For negative materials, exposure is almost always based on an important shadow value, which ensures adequate exposure for the low values (the opposite for transparency materials). Reading the mid-tones and higher values is useful for those who adjust their development times for the contrast range of the scene or for simply knowing what one has to deal with in terms of contrast range when making a print.
One of the real advantages of spot metering, especially for black-and-white photography, is using different readings to better imagine how the tonalities of the finished print will look (what Ansel Adams calls "visualization"). Developing a sense of how things will end up allows more expressive decisions to be made at the time of exposure (e.g., adding filters, adjusting exposure and/or development, etc.).
One mistake many make with spot meters is to take readings of high and low values and then average the readings to end up with a final exposure. This totally ignores the importance of shadow values to proper exposure and results more often in underexposure than using an averaging meter intelligently. If you've got a spot meter, just base your exposure on a shadow value (or highlight value for transparency materials) by deciding where to place it to get the detail you desire and figuring out how to do that with your metering method. Again, experience is key in learning how to do this well.
If you want to explore Zone System visualization, then take a lot of other readings of the scene and see if you can imagine how they will look in a final print. If you want to adopt Zone System development controls, then the range of tones in the scene and where you want them to be will determine what development scheme you choose.
In any case, the old Kodak rule of thumb applies when refining your methods: If your negatives are consistently underexposed, rate your film slower and vice-versa. And (primarily for b&w materials) if your negatives are consistently too flat, increase development time, and vice-versa.
The devil is in the details, but letting the basic principles be your guide will get you going in no time.
Best,
Doremus
