Photographers use light meters to ensure they get the right exposure the first time around. They are still in use by many photographers despite the advent of digital photography. Readings taken of the incidental and reflected light of an object are translated into shutter speed and aperture size on the camera. They can also be used when using a flash gun by setting off the flash as you take the reading.
Take a light reading by holding the meter in front of the object you wish to photograph. Push the button on the side of the meter to engage the display needle. When you let go of the button, the needle will stop in its current position on the scale. This is the light reading for the ambient light and can now be interpreted.
Set the film speed by rotating the film speed selector dial until the ASA number matches that of the film used. The needle points at a number used to work out the shutter speed and aperture settings required for a correct exposure.
Rotate the outer dial until the red V shape points to the number indicated by the needle. Within the cutaway of the dial, you will see the acceptable camera settings for the level of light shown in "F" numbers and shutter speeds. For example, an aperture and shutter speed reading of f5.6 1/250th requires the camera's aperture to be set at f5.6 and the shutter speed to 1/250th. The camera should now take a correctly exposed photograph.