We recently received an email from a student who was struggling to understand the difference between in-camera and handheld light metering.
For starters, it’s important to understand that there are two different ways to measure light in order to achieve a properly exposed image. You can measure the light reflected by the subject, which is how the light meter in your camera works, or you can measure the light falling on the subject (which we also call incident light metering) using a handheld light meter at the subject position.
An important factor to understand is that meters are calibrated to measure a neutral gray tone. A reflected light meter “assumes” that the subject is reflecting the same amount of light as a Gray Card would reflect. However, if the subject is bright white and shiny or black with a non-reflective matte finish, the meter will give a false reading which the photographer will then have to correct.
For simplicity, the following examples only mention opening or closing the aperture, but you could achieve similar results by changing ISO and/or selecting a different shutter speed.
With the camera’s reflected light meter, if the subject is bright white the photographer must select an aperture approximately 2 stops larger than the meter indicates. That’s because the camera “thinks” it’s looking at neutral gray, not bright white, so it’s going to indicate an aperture which will expose the scene as gray, not white. Imagine photographing a snowy winter scene in bright sunlight, or white sand dunes in the Sahara. If the camera’s meter indicates f/16 as the correct aperture, you may have to open the aperture to f/8 to get the snow to look white instead of gray. However if you are using a handheld incident light meter, it should give you an accurate reading of the bright sunlight illuminating the scene, and no adjustment will be required. The incident light meter should indicate f/8 in the same situation where the camera’s meter might indicate f/16. That’s because it’s reading the sunlight directly, not the light reflected by the subject.
If you are photographing a darker-than-neutral gray subject – for instance a black bear in dark, deeply shaded woods – the camera’s meter may give you a reading that will overexpose the subject. That’s because black bears reflect less light than a Gray Card, but the camera still “thinks” the scene is a neutral gray shade. So if the camera’s meter indicates f/2.8, you may have to select an aperture of f/5.6 instead to get a properly exposed image. Again, a handheld camera should give you an accurate reading of f/5.6 because it doesn’t care about the subject’s tonal value or reflectivity, only the value of the light illuminating the subject.
The same principles hold true with portraiture. A light-skinned blonde bride in a white wedding dress posed in sunlight with a bright-white wall behind her will probably be metered incorrectly (underexposed) by the camera’s reflected light meter while a dark skinned groom in a black tuxedo posed against a black curtain will likely be incorrectly metered (overexposed) by the camera. The photographer should understand this and increase or decrease exposure accordingly if they are relying on the camera’s meter, or use a handheld incident light meter for more accurate results.
In-camera metering can be further complicated depending on the metering mode the photographer chooses. Spot Metering will take a reading from a small “dot” in the frame. Average Metering, including Center Weighted Average and Matrix (Nikon) or Evaluative (Canon) modes will measure light from the full frame and average out the bright and dark areas. The course text in Lesson 2.3 suggests Spot Metering from the subject’s cheek, just below the eye, so you are exposing the image for the best skin tones.
One more note – either meter normally reads only the ambient “constant” light. You need a meter with Flash Metering capability to measure the instantaneous burst of light from a flash or strobe.