Whether you’re shooting a fully programmed automatic exposure or using modes such as Aperture Priority and Shutter Priority, or even if you’re setting exposures manually, in all cases you’re relying on your camera’s through-the-lens light metering capabilities to determine the correct exposure.
A DSLR’s TTL meter measures the amount of light being reflected from a scene into the camera. This metering approach is accurate, and allows for subtle composition changes to result in exposure adjustments as needed. The problem, though, arises when you don’t have your TTL meter set ideally based on the situation in which you’re shooting. With the wrong metering mode, exposures may change when they shouldn’t, or they might not change when they need to.
No matter what manufacturer or model of camera you’re using, you’re likely faced with variations on a few of the same basic metering modes: spot metering, center-weighted metering, or matrix/evaluative metering. Here’s how they work, and how best to put them to work for you.
The most precise metering mode is spot metering. It’s precise, literally, because it measures the light from just a small spot in the middle of the frame. This works just like a handheld spot meter would, with a better meter having a tighter spot—maybe a 1-degree or 3-degree circle. Point your camera at the subject, and know that the spot meter will give you a reading for that and only that spot. On some cameras, when you change the active focus point the spot meter will use the new point as the center of the spot. This way, the meter correlates to the spot being focused on. Very handy! The spot meter is especially useful when a subject may be backlit, or in other high-contrast scenes where you want to be very deliberate with your metering. The challenge, of course, is when you’ve got your camera set to spot and you’d rather it took a broader look at the frame and based the exposure on that.
Center-weighted metering can be thought of as a bit of a hedge between an overall evaluation of the scene and the focused metering of the spot. It’s like a bigger spot meter, which also factors in the rest of the frame. Center-weighted metering still places emphasis on the center of a scene like a spot, but the area being metered is larger. It also takes a look at the overall meter reading for the frame, and then blends the two readings together. That’s why it’s center “weighted” after all. The center is given preference, but the edges of the frame are considered too.
The third metering mode, Matrix or Evaluative metering (depending on the camera manufacturer) could effectively be called Averaged metering. This mode takes a look at the whole scene in the viewfinder, and bases a light reading on hundreds of segments within the frame, and averages everything it sees. This is useful for scenes that are low contrast, with flat, even lighting. Many DSLRs are even smarter than this, and use multi-zone variants to measure the light in specific places of the scene, even comparing them to a database of existing images, in order to make a more educated guess as to what the correct exposure might be. Also factored in to help make more accurate exposures? Focus distance and focus point, for sure. Matrix metering is highly accurate in a variety of situations, and should probably be the metering mode used for most general-purpose photography needs.
To change metering modes, look for the button with the metering mode icon—a partially closed rectangle with a circle in the center. Simply press it and use the scroll wheel to toggle from one mode to the next. The currently active metering mode is displayed by a variation of that metering mode icon, which appears on the LCD display of many DSLRs.
It’s also important to understand what all of these camera modes actually “see” when they examine a scene. They are assuming that any object in a scene is of average reflectance—namely, 18% gray. Knowing that your camera assumes that everything it sees is middle gray, you can anticipate when and how it’s going to provide misguided meter readings. For instance, in a particularly bright scene, knowing the camera will assume it’s actually darker might encourage you to adjust the exposure compensation to overexpose by a stop, counteracting the meter’s tendency to render a bright scene underexposed. Conversely, when photographing a black cat in a black hat in a dark room, be prepared for a camera that’s going to overexpose dramatically. Exposure compensation can be applied quickly and easily to compensate for scenes that don’t generally adhere to the 18% gray average. In this way, you can take steps to help the camera meter correctly and deliver the appropriate exposure recommendation every time.