There are common situations when the light conditions force you to override the metering system of your camera. In those conditions you know that the indication by the metering system of the camera is off to overexposure or underexposure your main subject. That it is the case when you shoot with the light source behind your subject, or when the subject and the backgrounds luminosity are separate by a very high contrast (one of them black, the other almost white, e.g.). The decision here is of two options: should I use spot metering, pointing to the subject face or any important part of it to get the correct exposure indications? Or should I set the Exposure Compensation (EC) mechanism to override by one or two stops the camera indications? Let’s see the concrete situations and alternatives.
First, I must indicate that it is my preference to use spot metering to solve those situations most of time. The problem to be solve is precisely of that: the camera Matrix/Evaluative metering mechanism is incapable to get a good measure by the unusual light conditions. Clearly then, I choose the more convenient spot metering to get a direct measure of the main subject, leaving all the environment over or under exposure. In rare occasions there is more than one subject. In those situations you have to select a main focal point, and hope the other subject to be equally clear and visible.
The EC by any stop you choose, presumes a very rare conditions. Obviously, if you are using Manual (M) mode, the selection is missing. In Manual you just adjust the Aperture, the Shutter speed or the ISO to get the exposure you want, no matter the light meter indicates.
In Aperture priority or Shutter priority the situation is the similar: why don’t you adjust the manual setting overriding what the metering indication dictate? Only when you use a complete auto mode (Auto or Program mode) you want to forget about settings and allows the machine to run your exposure selections. In those settings, you use EC to override whatever the camera choose, because no dial movement for the aperture or shutters speed will override the selection.
But stop. This is not the end of the story. As the spot metering is based in the selected focus point of the camera, you have to be precise when focusing to get the correct measures. With moving subjects it’s almost imposible 100% of the time: one shot good, the bird fly, you lost it, then you get the wrong spot, the bird is too dark, etc. In those situations you can’t be confident to point always to the correct spot. You prefer a more «general» system that allows good exposure results, not depending of the speed of your eyes and hands. EC is the best decision for those scenarios: subjects in motion in high contrast scenarios.
The main objection about those options (spot metering or EC) is regarding the irregularities you will find in metering systems of most cameras. They point, not to the matrix/evaluative system, but the spot metering option. I’m not in the position to discuss that without a real experiment, but these allegations are a little less than useless. Of course, every camera is different! That’s precisely the point when you decide to go full frame or a crop body, or when you go for Canon or Nikon. You expect different results, based on the character of the selected camera. That is the same regarding the metering systems. Each manufacturer use a they own technology and algorithms. The camera and the lens you use rein. The important thing to remember is that the differences produce similar results (good exposure indicators) must of the time, and are easily explained by the inclinations of the whole camera ecosystem: big dynamic range, big color range, sensor quality and size, type of lenses, and so on.
As a final point, I programmed one of the configurable buttons in my camera to select spot metering whenever I want change the metering calculations . This allows me to change from matrix/evaluative metering to spot in an instance, without leaving the viewfinder, and get the exposure measures I think the light situation demands.