Measuring Light
Automatic and point-and-shoot cameras usually have built-in light meters that will automatically measure the light's intensity and set the aperture and shutter speed to match it. These light meters are very good at giving you the right exposure in almost all situations. Your camera might not have a built-in meter, or the one it has might not work very well, in which case you'll want to use a handheld light meter. This is also a good option if you want to measure the relative brightness of different parts of a scene.
Light meters are about the size of a cell phone. They contain a light-sensing chip or cell, plus electronics that translate light intensity measurements into a display showing the correct aperture and shutter speeds. They meter incoming or incident light, reflected light, or both.
Incoming light, or incident light, is the light that naturally falls on a subject. To measure it, you put the light meter exactly where you plan to aim your camera. A light-sensitive chip, sitting inside a white translucent dome that looks like half of a ping-pong ball, measures the amount of light hitting the dome.
If your camera is giving you accurate exposures most of the time, you don't need a handheld meter; it will slow you down. On top of that, if you don't use it properly, your exposures may be less accurate than if you relied on your camera's meter.
Metering incoming light is a great way to preset shutter speeds and f-stops for action scenes. All you have to do is meter the scene before the action takes place. The meter will give you an f-stop and shutter speed that will correctly expose an average subject in the light you measured. If your subject is much darker than average — such as a black dog — open your aperture a stop or two from the handheld reading. If the subject is very light — such as a polar bear — you might stop down one stop or more from the incident-light reading.
This metering mode measures the light reflected from the subject. This is how all automatic cameras work, TTL or otherwise. Aim your handheld meter at the subject. If direct sun is hitting the metering cell, cast a shadow on it. The reading will be a combination of the amount of light hitting the subject, how much light bounces back from the subject to the meter, and how much of the scene the subject occupies. This is generally an accurate way to meter and set your camera.
Sometimes, however, conditions can make a reflected meter reading inaccurate. Here are some situations you may encounter and the adjustments you can make to correct them:
Subject reflects light directly (such as a window or a shiny surface on a building exterior). Get closer and either take an incident reading in the same light as the subject or move to a spot where there is no direct reflection.
Subject is a very small part of the scene (such as an actor in a spotlight on a dark stage). Get closer and use a spot or incident light meter.
Subject is backlit. Get closer and use a spot meter, an incident meter, or open up one to two stops from the initial reading.
Subject is dominated by black clothing. Shoot as metered if the subject is the clothing itself, or take an incident reading if the subject is the person wearing the clothing.
Subject is dominated by white clothing (such as a bride wearing a wedding dress). Take a reflected reading of the dress only and open up one or two stops, or use the setting given by an incident meter held at the bride's position.
Subject is part in shade and part in sun. The reflected meter should give an accurate reading if the meter is reading both the light and the dark parts equally. You might want to take incident or reflected readings in the shadow and the brightest portion, then set the camera halfway between them.
