External light meters are commonly used by studio and portrait photographers.
A light meter is a device that helps photographers measure how much light there is in a room or scene so they can adjust their cameras appropriately. Nearly all professional-level cameras have built-in light meters, but hand-held ones can be more precise and are better for certain situations.
A built-in light meter is visible as you look through the viewfinder of a camera. This type of meter measures all of the light in the scene and shows you what is basically the average. Hand-held meters have the advantage of being able to ignore the subject's surroundings and only measure how much light is falling on the subject.
Instructions
Built-in Light Meter
1. Point the camera at your subject and frame the scene the way you want it to be in your photo. Zooming in and out or moving the camera could affect the light meter's reading. If your subject is back-lit, you may want to zoom in on your subject while you read the light meter and adjust the camera settings.
2. Look through the viewfinder and push the shutter button down halfway. This will both take a light meter reading and, if you are in autofocus mode, focus the camera on the subject.
3. Find the light meter. The exact location depends on the brand of the camera, but it is usually at the bottom of the frame as you look through the viewfinder. It may also show up on an LCD screen on the back of the camera body. It is usually a row of tic marks or dots with a zero in the middle, a negative sign on the left, a positive sign on the right and sometimes a couple of numbers in between.
4. Look for a larger vertical line somewhere along the scale showing where the current light reading falls. If it is on the left of the zero, closer to the negative sign, there is not enough light and your photo will look dark. If it is on the right/positive side, there is too much light and your photo will look washed-out. You can make it move by adjusting the camera's shutter speed and aperture.
Hand-Held Light Meter
5. Turn the meter on and adjust the ISO setting to match what you are using on your camera. ISO, or film speed, indicates the film's (or, in digital photography, the sensor's) sensitivity to light.
6. Hold the meter up to your subject. If you point the meter's sensor toward your subject, it will measure the reflected light. If you hold the meter directly in front of your subject and point it toward your camera, it will measure how much light is actually falling on the subject. This is called incident light reading and is the more accurate method.
7. Press the trigger or "measure" button to take a reading. The aperture and shutter speed that you will need to set your camera to will appear on the LCD screen. Some meters will allow you to specify one of them beforehand and only adjust the other.
Tags: your subject, much light, light meter, measure much, measure much light, through viewfinder