Effective Spot Metering
You set the aperture, shutter speed, and ISO and you’re ready to expose your photo, but you failed to do one important step: The first thing you should have done was to meter your photo! Many photographers shy away from using the spot metering method, but it can be a useful tool if you keep a few key points in mind.
When using spot metering, your camera uses only a small area of the image to determine its brightness; the surrounding image areas do not influence the metering at all. Typically the active area is a small circle in the center of the viewfinder or monitor; this area generally comprises two to five percent of the total image area. Such a specific measuring area allows you to meter a specific part of your image very precisely.
A couple classic scenarios lend themselves to spot metering: when the brightness of your main subject differs greatly from its surroundings, or when large bright or dark areas or sources of light are located within your image. Such conditions skew and confuse the other two metering methods (center-weighted and matrix), producing an undesirable exposure. But you can use spot metering for other subjects as well. The advantage of using this method is that your camera will automatically produce the right combination of aperture, shutter speed, and ISO with a high level of accuracy. Also, when exposing your photo manually this method can quickly produce very accurate points of data, which you can use to manage difficult lighting conditions.
The disadvantage of spot metering, of course, is that you need to pay close attention to what you are doing and you need to know what subjects to meter to avoid failed exposures. With such a specific tool, a bit of inattention or just a tiny slip of the camera will produce a completely different exposure.
If you use spot metering in combination with one of the many partial or fully automatic exposure modes of your camera, you should get in the habit of using the exposure lock. Otherwise you’ll always end up metering the exposure for the center of your image even if that is not where your main subject is located. Another option is to set your spot metering to occur at the position of your active autofocus frame, if your camera offers this particular function.