Your camera has priority modes for aperture and shutter, but it also has a priority mode, of sorts, for the metering. Metering is when the camera reads the light of a scene and “decides” (based on the priority mode it’s in) what settings to change to create a correct exposure. There are typically three metering modes: spot, center weighed, and multi and/or “average”. In spot metering, the camera uses the center of the scene as the exposure sample, so whatever you point the center of the lens at, is what will get properly expose. This is usually about a 2%-5% area in the center. Center weighed is like spot, but the area is a bit larger: up to 30% from the center. I combine multi and average (from this point on, I’ll refer to them as “evaluative”) because, though they are evaluated differently, their goals are to average out the scene and make sure that the whole scene is correctly exposed.
So, why have several metering modes? Why not just have evaluative since this guarantees correct scene exposure? Well, for one, sometimes the scene makes the subject too difficult to expose so you need to give priority to your subject. The other is to take advantage of the camera’s “zone system” (a concept developed by Ansel Adams). This is also known as the camera’s dynamic range. The zone system is a way to identify zones in a scene that range from pure white to pure black. Typically, you want to avoid blowing out highlights (pure white) or losing details in shadows (pure black), but it can be used creatively. Also, you may not have any choice if you want your subject to be properly exposed.
There are ways to evaluate the dynamic range of a camera. Know that the bigger the range, the better. This range is measured in “stops”. A “good camera” would have a dynamic range of 10 stops. This is also the range that software like Nik Silver Efex Pro uses to help you identify the zones in your photographs, as with the following photo. Notice that as the curser is hovering over the zones, the software highlights the areas that match (indicated by the arrows).
I shoot exclusively with spot metering. This allows me to guarantee that my subject is properly exposed from the onset. If I need other areas in the scene to show more or less, then I do that in post-processing. Spot metering also allows me to take advantage of the camera’s zone system to create more dynamic images. Images are that are shaped by the light as opposed to the artifacts that make up the image. It allows me to put into practice the chiaroscuro technique. Chiaroscuro is Italian for “light-dark”. It’s a characteristic of a photograph or a painting where light creates strong contrasts as opposed to colors. This is depicted in the following photograph of a man on a smoke break.
In this photo, I spot metered on the man’s face and allowed the camera to map out the zones in the rest of the scene. This next photo demonstrates how spot metering can create a dramatic portrait using directional light.
The next two photos of a table with lamps, depict the difference between shooting using spot and evaluative metering. The photo on the left is focused and properly exposed for the lamp in the center of the frame. Using spot metering, you can draw attention to the subject or area of interest. With evaluative metering (right photo), the emphasis is lost. I actually shot this using only spot metering. I used Adobe Lightroom to simulate evaluative by selecting “Auto” on the Exposure slider, for demonstrative purpose. This is a similar approach to what the camera does it, but not exactly the same.
This is not to say that evaluative metering is not useful. It just depends how you want to light the scene. Also, you can always balance the exposure in post if shooting digital. These techniques really come into play when shooting JPEG or film, since they are more difficult to modify, but also because you want to take advantage of the light at the scene and it also reduces the amount of work you do in post.