The spot metering IMO has better use than just taking a reading of several areas and then "averaging" them. If the "contrast" of your scene is larger than the "dynamic range" of your film, such an average reading will lead to an image which has a certain amount of blocked shadows and a certain amount of burned highlights, and that is not necessarily what you would have wanted. You might have wanted to avoid burning highlights first, and let shadows fall, and block, where they may. In this case, spot metering is the solution to the problem.
The superiority of spot metering is in being able to exactly "place" "spots" of your scene on the characteristic curve of your film and being able to visualize the results, especially when your blanket is shorter than what you would need to cover the entire scene.
A typical example would be to use spot metering when taking an outdoor high-contrast scene with slide film. With slide film, what you want to avoid is to burn highlights. Also, you want your highlights to be "highlights", i.e. near-whites. That is, you want to place your highlights on the "near-whites" zone of your film.
You spot-meter the highest light zone that you want to be "readable" in your slide (that would be within "zone VIII" in zone parlance). The immediate reading of the spot light meter would render this spot as middle grey. You want it to be whitish, so you open by 2 and 1/3 EVs, (supposing your slide film begins burning highlights shortly 2,3 EVs above middle grey). If your spot light meter reads 1/125 f/22 (measured exposure) on the highlight zone, you set your camera at 1/125 f/10 (aperture at f/8 and then closed 2/3 toward f/11). That is the "calculated" exposure for the high lights.
You now "scan" the scene with your spot light meter concentrating on the lowlights, they will begin blocking at 2,7 EVs below your calculated exposure. In this case, lowlights will begin blocking on "spots" where your spot light meter reads 1/125 f/4 (reading for middle grey on film). Beware, those spot reading 1/125 f/4 is not going to turn out middle grey in your slide, that's going to turn out quite dark (somewhere in zone II in zone parlance) and is where your shadows will begin to "block" on film. (That is, because you are taking the picture at 1/125 f/10 not at 1/125 f/4).
By scanning the scene, you can visualize which shadows will "block" on your film and decide if the image is going to result in something you like.
It goes without saying that if you see a too high amount of blocked shadows, or you see that spots with meaningful shadows where you would like to see details will block on film, you can:
- give up taking the picture and wait for a different light condition, if outdoors;
- adjust lighting, if indoors in studio;
- examine whether to use fill-in flash, reflectors etc.
- exclude some lowlights zone from the composition, study a new composition;
- exclude some highlight zone from the composition, and recalculate exposure to open up shadows;
- accept some highlight burning if that is acceptable for the final result;
Any kind of averaged, or even worse "matrix" reading will not give you a clear idea of where the high lights and the low lights of your scene "fall" in the characteristic curve of your film.
If you use negative film, such exact "placing" is not vital (you have room for compensations in printing, and in general negative film have much higher dynamic range, you will get plenty of details in both high lights and low lights).
If you use slide film, correct "placing" of highlights for difficult, contrasty scenes is of paramount importance, and spot metering is your best friend.
Learning to properly use a spot light meter with slide film will save you from a lot of bracketing
Fabrizio
EDIT for clarification. If the scene has highlights indicated by your spot meter at 1/125 f/22 and low-lights indicated by your spot meter at 1/125 f/2 and if you just "average" the two extremes, you will obtain something around 1/125 f/6,7 and by using that value with slide film you will grossly burn your highlights. By using the method outlined above, you will expose at 1/125 f/10 to preserve correct highlight rendering and, before taking the picture, scan the lowlight zone knowing that wherever the light meter reads less than 1/125 f/4 you will get a blocked shadow. After this "scanning" you will take the picture or adopt one of the corrective actions.
