If you look around the room you’re sitting in now, can you imagine how your video camera would see it? Perhaps the lights are nice and low, giving the place a cozy or romantic feeling. Try shooting in a room like that, without adding lights, and you’ll end up with a lot of murky shadows and almost no detail–not very cozy or romantic. The room is not lit for video, so you can bet the camera will not pick up all the details you’re seeing. What can be worse is when the camera picks up details that you don’t even notice. If you’re sitting in an office for instance, lit by rows of fluorescent lights on the ceiling, you might not notice the green tinge the fluorescent lights give everything, but the camera will. You might not notice those unappealing shadows under the eyes and chin, caused by the light source being at an angle too high above the subject, but the camera will.
As noted earlier, the camera imitates the eye, but it doesn’t work in exactly the same way. The light reflecting off of your subject is the same whether you are looking at it with your naked eye or through the viewfinder of your camera. However, the iris in your camera and the iris of your eye react to it differently. Both narrow as a reaction to objects that reflect a lot of light. Both widen as a reaction to objects that reflect very little light. How much the aperture or iris opens depends on the amount of light needed for the lens to focus properly and for the image to be registered on the light sensitive material behind the lens, i.e. the retina, film or charged coupler device (otherwise known as a CCD chip, the imaging sensor used in video cameras). The difference between how eyes react to light and how cameras react to light lies in the amount of light needed for an image to register, which changes depending on which light sensitive material (retina, film, CCD chip) is being used.
Your eye can take in a relatively large amount of light or a very small amount of light and still register an image accurately. This is not so for film or videotape. There is a prescribed amount of light, neither too much nor too little, which film and CCD chips need in order to record an image. Scenes and objects being videotaped must also fall into this “Goldilocks” zone. If they are too bright or too dark, the imaging sensors will not handle them well. Very bright objects come out like glowing blobs with no details, and very dark objects simply don’t show up at all.
In order to stay within the Goldilocks zone, video camera light meters are designed to treat everything as if it’s mid-toned, setting the exposure so that objects in its field of view appear to reflect approximately eighteen percent (more specifically ten to eighteen percent) of the light that hits them. Why eighteen percent? Eighteen percent is the amount of light that gets reflected when you look at a shade of gray that is right at the halfway point between absolute white and absolute black. More importantly, eighteen percent is a common amount of reflectance. It is the amount of light reflected when you look at an average landscape. Anything at ten to eighteen percent reflectance is considered mid-toned– grass, carrots, robin eggs, etc. We’re used to looking at a world and at objects that are only moderately bright, i.e. mid-toned. And so, it makes sense to make ten to eighteen percent reflectance the standard for most light meters, including the one that controls the automatic exposure setting in your camera.
The problem is, not everything is mid-toned. Therefore, when using automatic exposure, anything tha’s relatively bright, reflecting more than this magical eighteen percent, will come out too dark; relatively dark objects, which reflect less, will come out too light. The farther away the object is from eighteen percent reflectance, the less accurately the image will be recorded. Adding or reducing the light in the scene is a way to correct for objects that are not mid-toned. But, the auto-exposure will still treat them as if they are mid-toned, so, you’ll need to adjust the exposure manually in order for lighting adjustments to work.
The subject of the next page is manual exposure control.