The Basics of Digital Photography: Digital Camera Metering Modes

One of the keys to making great photos with any camera, digital or film, is to have a good understanding of how light metering systems work and in knowing how to select the best mode to use in any given shooting scenario. The Exposure Triangle that we covered in chapter one of "The Basics of Digital Photography" introduced you to lens aperture, shutter speed, and ISO. Until now, you have been shooting using one of the programmed modes or in Aperture Priority Mode or Shutter Priority Mode. To start using the Full Manual Mode you need to understand how to use your cameras light metering modes and you need to know how to select the best metering mode to use for the type of photography that you are doing.

The three most common metering modes that are available on most digital cameras are the Matrix Metering Mode, the Center Weighted Metering Mode, and the Spot Metering Mode. Each of these metering modes have their strengths and weaknesses and the key to making great pictures is understanding those strength and weaknesses because only through understanding them will you know which mode to use in a given shooting situation. These common metering modes are available on all DSLRs but the way you access these menu items vary from camera to camera so once again dig out the manual that came with your camera and read up on its metering modes and how to access them.

We'll begin by defining each of these metering modes.

Matrix Metering Mode, in this mode the camera divides each scene up into a number of discrete segments, the numbers of discrete segments vary from camera to camera. When set for matrix metering the camera takes separate reflected light readings from each of the zones in the matrix. The camera then selects an exposure setting that is best for the majority of the frame zones sampled. Matrix metering, also known as multi-segment metering or multi-pattern metering, is the best metering mode to use most of the time because it assures you the best overall exposures under most conditions.

Center-Weighted Metering is a compromise between Matrix Metering and Spot Metering. When operating in the Center-Weighted Metering Mode, the camera meters the light reflected from a small area around the center of the frame. This mode is especially helpful when shooting portraits. This is the default-metering mode on most DSLR cameras.

Spot Metering is just that, the camera meters the reflected from a tiny spot area in the exact middle of the frame. This mode is especially useful when shooting high contrast pictures. For example, let's say you want to shoot a picture of a girl sitting in front of a window in a dimly lit room. If you shoot using center-weighted or matrix metering you are going to end up with a silhouette because the camera is going to expose for the brightly lit scene outside the window. The trick here to properly expose for your subject is to switch to the spot metering mode and meter off your subject.

Before we become more involved in learning how to decide which metering mode will be best for the situation you in, you need to understand a little more about how a cameras built in metering system meters light. Today's TTL (Through The Lens) camera light meters have many advantages but they also have one fundamental disadvantage, they can only measure reflected light. TTL light metering system can't measure "Incident Light." The problem is caused by the fact that all objects in the real world don't reflect the same percentage of incident light striking it. Because of this in-camera metering systems are calibrated to measure light reflected by a mid-gray object. Objects that fall above or below this midrange of gray are read incorrectly and thus exposed incorrectly. Handheld light meters, on the other hand always meter the light correctly no matter where they fall on the gray scale and that's why every studio photographer still uses one. If you are serous about photography, you will want to invest in a handheld meter, preferably one that reads both incident and reflected light as well as functioning as a flash meter. These meters can become quite expensive but with a little shopping around you can get a decent one for around $150.00.

Handheld light meters can be as easily fooled as cameras built in light meter if you use it to measure the reflected light from the subject. The good news is that quality handheld light meters come with a white dome shaped device, an "Invercone," that fits over the light meter's sensor converting it into a "Incident Light" meter. To use an incident light meter you must place it right next to your subject with the Invercone facing the light source. Outdoors the Invercone would be pointed at the sun; indoors it would be pointed at the source of artificial illumination. In some situations, like when shooting landscapes, the photographer has no choice but to shoot using a reflective light reading.

Exposure Compensation. This is a good point to introduce the new photographer to the concept of Exposure Compensation and how to use it. Most DSLR cameras have a built in feature called Exposure Compensation. Again, refer to your camera user's manual to find out if your camera has this feature and if it does to find out how to access that menu on the menu system. Exposure Compensation works when shooting in one of the cameras programmed modes. In the Aperture Priority Mode, or want to use a positive setting when the background is more brightly lit than your subject in the Shutter Priority Mode. Most cameras allow you set the Exposure Compensation for a ± 5 EV (Exposure Value.) As a general rule of thumb you will and a negative setting when your subject is more brightly lit then the background. The best way to learn how much compensation to dial in is to experiment with your camera under different shooting conditions.

In chapter three of The Basics of Digital Photography, we'll take a look at "White Balance" and the concept of "Color Temperature."