Modern DSLRs are incredibly clever machines, with built-in metering systems that measure light reflecting from the subject. This is standardized so that the camera sees your focal point as mid-grey. (Mid-grey is 18% grey, which is exactly halfway between pure white and pure black.)
Although this is relatively accurate most of the time, the camera occasionally gets it wrong. Fortunately, most cameras come with metering options that allow for better readings across a greater range of subject lighting and reflective combinations. These options work by assigning weighting to different areas of the image, which helps provide a more accurate exposure. Here’s a look through these different metering modes.
Both of these terms are used to describe the default metering mode on nearly all digital cameras. If your camera is in automatic mode, this is usually the only metering mode available. Evaluative metering works by measuring light across the whole picture frame to average out an exposure. It should be noted that this mode is biased toward the area around your chosen focal point, by assessing the subject’s position in the viewfinder. It will also record the brightness of the general scene, front and rear lighting conditions, and the orientation of the camera (i.e., if it’s in portrait or landscape orientation) to work out the final exposure. This is the most widely used metering mode, but the results aren’t always predictable and can sometimes get the exposure slightly wrong.
It’s a little known fact that center-weighted metering actually gives the most predictable results in metering terms. The reading is strongly biased toward the center of the viewfinder (hence the name), but will, in fact, measure light across about 80% of the picture area. This mode is also clever in that it doesn’t take focus into account, resulting in the same averaging pattern for each shot. Center-weighted metering is particularly useful for backlit subjects, as you can make sure that the metering focuses on the subject rather than the strong light. Pro tip: you can fool the metering by placing the center point over your subject and then recomposing with the shutter still half-pressed. You’ll need to shoot within 12-15 seconds before the metering system resets.
This very specific form of metering measures the light over the center 5% of the viewfinder in a small circle form. It’s a fairly tricky mode to master, but is incredibly useful if you have a very small object that needs to be perfectly exposed. For this reason, spot metering is often used by still-life photographers. Additionally, if you can identify the closest match to 18% grey in the photo, you can then meter for this to ensure that the rest of the object is perfectly exposed. Some professional DSLRs have the ability to take multiple spot-meter readings from a scene to take an average reading and set the exposure accordingly.
You won’t find this option on all digital cameras, but it’s a very useful option if it is there. This mode meters for a slightly larger circular central area than spot metering — around 15%. Along with center-weighted metering, it’s very useful for using in a portrait where the subject is backlit. Do take care with skin tones though; if the skin shade is vastly different from 18% grey, you’re likely to end up with some rather inaccurate exposures. Partial metering is also useful for still lifes if you’re working with slightly larger objects.
Knowing your metering modes makes it far easier to bring accuracy to your shots and fine-tune the exposure of your photographs. While the in-camera metering system will never be quite as accurate as using a handheld meter, knowing the different modes provides an easy way to ensure you get the best out of your camera in changing lighting situations.