Most cameras have a general purpose-metering mode that’s will be called something like Matrix, Evaluative or Multi-segment metering. This is usually the default mode, and it sets the camera to look at the whole image frame to assess exposure. When it’s used, the camera sets (or recommends in manual exposure mode) exposure settings that it calculates will get the majority of the image looking right.
However, very bright or very dark areas can cause problems, and result in images that are under or overexposed. Thankfully, most cameras have a spot metering option that can come to the rescue.
The camera takes a measurement from a single point to determine exposure
What is spot metering?
Whereas multi-segment or multi-zone metering takes exposure readings from across the whole image frame, in spot metering mode the camera takes a measurement from a single point or spot. This area is often at the centre of the frame, but it may be linked to the active AF point. In some cases the camera’s menu gives you the choice to use the centre spot or the AF point - it’s important to know which point is in use.
Once spot metering mode is selected, your camera will set (or recommend) exposure settings that make the area under the spot a mid-tone. If the spot is positioned over a white subject the exposure settings will make it grey, similarly, a black subject will also be made grey. So the key to using spot metering mode is to position the spot over a mid-tone, or an area that you want to be a mid-tone in your image.
Use spot metering in conjunction with Auto Exposure Lock
Using spot metering
Some cameras have a button, dial or switch that allows you to select the metering mode, with others you have to use the menu.
Many photographers use spot metering in conjunction with Auto Exposure Lock (AEL or AE-Lock) as this enables them to select the area for exposure measurement, lock the exposure and then recompose the image before taking the shot.
To do this simply position the metering spot over the area that you want to be a mid-tone, then press the AEL button before composing the image and pressing the shutter release to focus the lens and take the shot.
Because the spot metering area is small you can take a meter reading from a specific part of a portrait subject’s face.
Alternatively, if your subject needs to be a mid-tone and you have the option, link the spot metering to the AF point and your subject should be correctly exposed as long as the active AF point is over it.
Spot metering is especially useful for portrait photography
When to use spot metering
Spot metering is useful in tricky lighting situations such as high contrast scenes or when the subject is backlit. It’s also useful when a subject is against a very bright or very dark background. It enables your camera to ignore parts of the scene that could confuse its multi-segment exposure system and concentrate on the most important area. It’s especially useful for portrait photography.