Of all the technical gremlins that can frustrate a videomaker, poor exposure has got to be the most common. Whether
you’ve used a camcorder a thousand times or only bought your first one last month, the chances are you’ve
experienced:
- faces that become black smudges against bright backgrounds
- overall brightness that changes wildly as you pan the camera
- burnt-out highlights, or shadows so black that you can’t see into them.
You can usually prevent exposure problems if you understand what your camcorder exposure system does, how it
works, what its limitations are, and how you can take command of it. We’ll run through these topics and then suggest
what to look for in exposure control when you buy your next camcorder.
Exposure Control Basics
The key to understanding exposure is the idea that the camcorder chip that forms the video image demands a
precise measure of light to do so.
Let’s call that amount of light "1X." Your camcorder needs exactly 1X light to form a high-quality image. Give it,
say, 1/4X light and the picture will be almost black. Blast it with 4X light and the picture will flare out to white.
In other words, the chip doesn’t care whether you’re shooting in a candle-lit dining room or on the beach at high
noon. Regardless of the amount of light available, the amount admitted must be precisely 1X to form a good quality
image–no more and no less.
The camcorder’s exposure system regulates two things: iris and shutter speed. The iris diaphragm in the lens
controls the amount of light admitted, while the electronic circuitry referred to (metaphorically) as the shutter governs
the amount of time the chip has to respond to the light. By manipulating the lens aperture (and sometimes the shutter
speed), your camcorder does its best to deliver 1X light to the image sensing chip regardless of lighting
conditions.
Automatic Exposure
Most of the time, the auto exposure system controls the amount of light reaching the camcorder’s CCD.
Here’s how it works. When you turn on the camera, special circuitry analyzes the amount of light hitting the chip
during the 1/60 second it takes to form an image. If that amount is greater than 1X, which is usually the case, the
circuitry calculates how much to "stop down" (close) the iris diaphragm that sits between the sensor and the light
source. Then it sends the appropriate command to the circuits controlling the servo motor of the diaphragm. The
motor closes the diaphragm down, light transmission falls to 1X, and the CCD forms a perfectly exposed image.
(Well, sometimes.)
Automatic exposure has obvious advantages. First, it relieves the videomaker of one whole category of stuff to
worry about while shooting. Also, it lets you shoot quickly in fast-moving situations because the system adjusts to
changing light conditions as you move around. If you’re gathering dramatic footage of the flames and smoky shadows
of a raging fire, you don’t have time to fuss with exposure settings.
On the other hand, automatic exposure control has two serious disadvantages: it’s slow and it’s dumb.
It’s slow because exposure adjustments require electro-mechanical operations, which take time. As a result, the
system often can’t react fast enough to changing light to maintain a steady exposure. Try panning as someone walks
out of the shade into bright sunlight and you’ll see a brief blast of overexposure until the system catches up with the
changed lighting conditions.
But the auto exposure’s sloth is not as hard to deal with as its dumbth (a wonderful word coined by
entertainer/philosopher Steve Allen). Camcorder circuits have only enough smarts to determine how much light is
hitting the camera chip. They haven’t the brains to decode the image that light forms, let alone determine which part
of that image to expose properly.
Stark Contrast
Which part? Why can’t the system properly expose all parts of the image? Because the chip can’t always handle
the contrast between dark and light areas.
"Contrast" is the difference in brightness between the lightest and darkest parts of the image. It’s expressed as a
ratio, such as four to one, meaning that the brightest point on the image is four times as light as the darkest.
Within the system’s contrast range, details in the light areas ("highlights") and dark areas ("shadows") are distinct
and readable. Above and below that range, highlights "burn out" or become blobs of pure white and shadows "block
up" or drop to solid black. Four-to-one is about as wide a contrast ratio as a video system can maintain.
Trouble is, in real-world scenes the contrast is usually higher–often much higher. For example, if you shoot a
backlit face against a bright blue sky, proper exposure might be f/4 for the face and f/22 for the sky. That’s a contrast
ratio of 32 to 1–eight times greater than the camcorder can handle.
So what’s a poor camcorder to do? Simple: it properly records those parts of the scene that fall within its contrast
range and lets the others burn out or block up.
Which parts fall within its range? That depends on two characteristics of the auto exposure system: which part of
the image it reads to calculate exposure and which part of the black-to-white spectrum it favors.
Measure For Measure
Some exposure systems merely average the brightness of the entire image and set the aperture accordingly. This
works well enough when the important image components are in the center of the gray scale.
But in our example of a shadowed face against a bright sky, this system would overexpose the sky and underexpose
the face, because neither element is a middle gray.
The next step up in sophistication is a system that sets exposure to favor the highlights. For psychological reasons,
viewers can tolerate opaque shadows much better than flaring highlights. By biasing the exposure toward the upper
end of the gray scale, this type of system protects the bright areas from burning out. The penalty is that more of the
picture is in featureless shadow. With this system, our sky would be perfectly exposed but the shadowed face would
be a black hole.
This problem led camcorder designers to think, hmmm: most videomakers put the important part of the
picture in the center of the frame, don’t they? Okay, we’ll make the exposure system measure only that important
center area.
Fine, except that this compromise perfectly exposes our shadowed face but renders the blue sky ghastly white.
The next step was "weighted averaging," in which the camcorder read the entire image but gave the center portion
extra importance.
Some camcorders have a "spot" or "highlight" metering mode which sets exposure based on the brightest
area of the image. This mode is handy when shooting a strongly-lit subject against a dark background. Highlight
metering will correctly expose a spot-lit performer, instead of turning her into a radiant ghost.
And so it went, with engineers designing ever more sophisticated metering compromises. But these compromises
don’t work perfectly, for a very simple reason: an automatic system can’t properly expose the important elements of
the image because it can’t know what they are. Only you can know that, which means that sometimes you have to step
in, roll up your sleeves, and take personal command of your exposure system.
Taking Charge
There are several ways to do this. At the simplest level, you can use the camera’s "backlight" button.
Here’s how: when you see that you’re about to shoot, say, a dark face against a bright sky, you press this button and
the lens aperture opens up (depending on the camera) one, 1.5, or two f-stops wider than the meter would
normally set it. The extra light admitted improves the exposure of the dark face.
The problem with this approach is that the backlit subject may not be the "correct" number of f-stops too dark. In
our previous example, the sky needed f/22, while the face required f/4. If the backlight switch opened the lens two f-
stops, the exposure would be f/11–two stops too large for the sky but still three stops too small for the face. In this
common situation, neither part of the image would be correctly exposed.
In short, the backlight button can help somewhat in some situations, but it’s really pretty inflexible. A second type
of compensation switch changes the area metered. At the normal setting, the light meter measures most of the image.
But when you press this button, the meter reads only the central portion. If you used this approach to compensate for
your backlit subject, chances are you’d achieve excellent exposure on the shadowed face.
More sophisticated cameras allow you to fine-tune the lens aperture. Some camcorders have an iris control set the
aperture at any position from fully open to fully closed. In manual mode, for example, the Panasonic AG-455 allows
you to change the aperture in very small increments, while a readout of the exact f-stop appears in the viewfinder.
Tips For Better Exposure
All right, so your camcorder permits you to control exposure, at least to some degree. That’s great, but first things
first. Before you start playing with camera controls, try to improve the lighting conditions by reducing contrast:
In making backlit shots, avoid shooting against the sky or other light backgrounds. This will help keep the
contrast range small enough to handle. The solution is often as simple as moving your subject or camcorder.
To eliminate exposure "lag" when a subject moves from a dark background to a light one (or vice versa), don’t
follow the move in one panning shot. Instead, stage the action in two successive setups, one shady, one sunny.
That way, you can cut between two equally well-exposed shots.
Outdoors, fill in shadows with reflectors. (For advice on how to do this, see Michael Loehr’s article "Outside
Lighting" in the October 1994 Videomaker.)
Indoors, use a camera-mounted light to fill in shadows. Or buy an inexpensive halogen work light on a floor
stand and bounce its light off a lightly colored ceiling for a general fill light.
Above all, pay attention to the contrast range of every scene you shoot.
The pros do this by checking the image on a well-calibrated monitor (except when shooting news, sports, and some
types of documentary), and you should too. The finest camera viewfinder made cannot show you the subtleties of the
image. For checking contrast, even a black and white portable TV will do–and you can get one for under $100.
When an external monitor is impractical, try this trick: look at the scene with your eyes scrunched up until your
eyelids are almost closed. This will reduce your eyes’ ability to handle contrast to about the level of your camcorder
and you’ll see the highlights and shadows the way they’ll appear on tape.
Use this system to size up the contrast in the scene. If it’s too great and you can’t reduce it, choose a different
background.
And when that isn’t possible, use your manual or compensation control to bias exposure toward the highlights.
Again, viewers accept inky shadows much better than eyeball-scorching highlight flares.
A Word About Light Levels
In some situations, the problem isn’t too much contrast, but too little light. Many camcorders will shoot in very low
light, but they pay a penalty for their sensitivity: low-light images look murky and grainy.
Why? In very low-light situations, the CCD can’t receive 1X light, even with the lens wide open. When this
happens, the camcorder boosts the wimpy signal up to 1X by amplifying it, exactly as you turn up the gain on your
stereo. It’s no coincidence that the control for this feature is called "gain." Some camcorders include a selectable "gain
up" switch. Others begin boosting the signal as soon as the light level drops below 1X.
The problem with this is that in beefing up the signal, the camcorder amplifies unwanted video noise as
well. That video noise takes the form of grainy or "snowy" images with muddy, inaccurate colors. The only way to
avoid video noise is to add enough light so that you don’t need the gain control.
Exposure And Shutter Speed
In normal shooting, you or your camcorder can manage exposure by changing the lens aperture, but the shutter speed
is also a factor. Remember, we began by saying that proper exposure depends both on the amount of light entering the
lens at any moment and the amount of time allowed for storing it in the CCD. That amount of time is called the
shutter speed, even though your camcorder doesn’t actually have a shutter. The term comes from comparing the
camcorder to its older counterparts, the still camera.
Many camcorders allow you to increase the shutter speed in order to sharpen the individual frames of fast-moving
objects. Each time you double the shutter speed you halve the time in which the chip can accumulate light. To
compensate for this loss, the camcorder opens the lens aperture one f-stop each time you double shutter speed. No
problem, as long as there’s ample light.
For instance, suppose you want to analyze the form of a basketball player by setting the shutter speed at 1/2000 of a
second. That’s one-fifth as much light as the chip would receive at 1/60 second, so the camcorder would have to open
the lens five stops to compensate.
But because you’re already shooting at f/4 inside the gym, your lens doesn’t have five f-stops left to open. To do
this, the exposure system would have to set the lens at a nonexistent f/0.95. Unable to achieve 1X by widening the
aperture, the camera would increase the gain instead, thereby reducing the image quality.
Your Next Exposure System
Though you probably won’t choose your next camcorder on the basis of its exposure system, these features
ought to be part of your buying decision. Here are some general tips:
Demand better than a take-it-or-leave-it backlight button. You need at least some type of iris control. In fact, if
you’re very serious about image quality, buy a camcorder that offers full manual iris control.
Look for a switchable metering area so that you can set exposure based on either the whole frame or the center
part of it.
In the store, check the speed of the exposure system by panning rapidly from the store interior to a much brighter
scene such as the store’s plate glass window.
Go for a switchable gain control. That way, in low light situations you can choose whether or not to amplify the
image (and its attendant noise).
And no matter which camcorder you choose, remember that auto exposure is just a convenience for quick-and-dirty
shooting. For the best image quality, you, the videomaker, should control exposure yourself.
Good shooting!
