A little background
From the mid-sixties, built in light meters became common in SLR cameras. In the digital photography through the lens metering has always been built in to the dSLR.
In a similar way, mobile phones, all have cameras and all have built in light meters by design. More recent designs are able to meter correctly by scanning the objects being focused on and metering for that element in the frame.
So the question “do you need a light meter?” seems quite pointless, but….
Reflected light versus Incident light
All cameras measure the “reflected light” from the scene in front of them, not the light in the scene.
Suppose we have a room with a black wall opposite a white wall and one light source. The brightness and color temperature of light is constant. If we point the camera to the white wall and take a meter reading then point the camera at the black and take another reading, we’ll get two different readings. But how can this be? The light in the room is constant and so the light falling on the black wall must be the same as the amount of light falling on the white wall.
The issue here is that the meter is reading reflected light. The algorithms in the camera assume that the light reflected by a average scene, in front of the camera, must average to approximately 18% gray. This is the value has been set through many years of experience and research and also the human eyes are most at home when the scene before them, averages out to around this 18% gray.
So when we meter reflected light from our camera, the white wall is calculated as too bright, the meter compensates by reducing the recommended exposure. Turning a white wall into a light gray wall. Similarly the black wall gets the reverse treatment, exposure is increased and so the black wall turns dark Gray.
What we need to measure is the light that’s falling on our two walls – light meter to the rescue. If you measure the light falling on the white wall, using a light meter and measure the light on the black wall. You’ll find that both readings are the same. This is the value you need to set on your camera (exposure/shutter speed combination).
When you use a light meter and with a little practise, your whites will always be white and your blacks will always be black