Image Stabilization: High-tech Help for Shakey Shots

We've all sat through video that shook, shimmied and jerked so horribly it should have been preceded by a "viewer discretion advised" graphic. In truth, everyone who has ever held a camcorder has shot shaky video.
To realize the benefits, and avoid the pitfalls of using image stabilization, we first need to understand how it works. There are two popular methods of stabilizing video, so we'll examine each of them separately.

Moving Pictures
Understanding how image stabilization works isn't very hard. To illustrate, we'll consider a story of two corrupt binocular salesmen: Jerry and Larry. These optical outlaws bilked hundreds of people $145 apiece for fake "super-stabilized" binoculars.
Jerry and Larry always set up their binocular cart in front of the same picturesque graveyard. When a victim tried their bogus binoculars, Larry would jump behind the cart and retrieve a poster-sized picture of the graveyard. He would then quickly position the picture between the victim and the graveyard.

With sharp eyes and fast hands, Larry would watch for small movements of the binoculars. If the binoculars moved a degree to the left, Larry would shift the picture to compensate. If they moved three degrees up and to the right, Larry would move the picture accordingly. To the person looking through the binoculars, the image was steady as a rock.

Though the technology is a little different, a camcorder's image stabilization system works in much the same way. If the camcorder detects unwanted motion, it shifts the recorded image to compensate. If you move the camcorder a few degrees to the right, the image shifts to cancel the motion. The result is smoother shots, up to the mechanical or electrical limits of the system.

Optical and Digital
Camcorders achieve image stabilization though two distinct ways: through digital processing or optical trickery. The digital approach is the more simple and cheaper to implement, as it requires no moving parts. Optical image stabilization adds complexity to the camcorder's lens, which translates to a higher cost. This explains why you usually find optical image stabilization on top-of-the-line camcorders.
Digital image stabilization (often called Electronic Image Stabilization or EIS) performs its magic using the CCD of the camcorder. First, EIS takes the central portion of the CCD image--roughly 90 percent of the sensor's output--and digitally expands it to fill the whole screen. This frees up the outer edges of the CCD and allows them to function as a buffer area. The camcorder can then move the smaller active image area around in any direction on the face of the sensor, until it reaches the physical border of CCD.
Imagine if your camcorder had a little joystick that would allow you to move the active image area around at will. If your camcorder were stationary, the result of moving the image area would look just like you were panning and tilting the camcorder. The camcorder itself isn't moving, but shifting the active area of the CCD makes it look like it is.
The reverse is also true. If you were to pan the camcorder to the left, and move the active image area in the opposite direction, the resulting video would look like the camcorder didn't move at all. If you could move the joystick quickly enough to counteract every jiggle of the camcorder, the resulting video would appear as if the camcorder were stationary. This is exactly how EIS works: just like Larry and his moving picture with the binoculars, EIS senses the camcorder's motion, and moves the CCD's active area to counteract the motion. See Figure 1 for a visual explanation of the EIS system.
Optical image stabilization achieves the same result by completely different means. Instead of moving a portion of the CCD image in response to camcorder motion, optical stabilization actually redirects the light coming into the lens. It does this with movable prism mounted near the front of the lens.
A prism is basically a hunk of glass with splayed surfaces. Light passing through the prism bends in proportion to the angle between the front side and the back. If the front and back of the prism are nearly parallel, the light bends by a small amount. If the angle between front and back is greater, the light takes a more pronounced turn through the prism.
Camcorders with optical stabilization use an ingenious device called a variable-angle prism. This prism is nothing more than two pieces of glass separated by an optically inert fluid, with a flexible bellows all the way around their perimeter. Small motors collapse or expand the sides of the prism, changing the angle between the front and back lens. This allows the camcorder to steer the light moving through the camcorder's lens. Couple these motors to an electronic circuit designed to sense a camcorder's motion, and the prism can counteract small movements of the image by steering the light in the opposite direction (see Figure 2).

See It, Feel It
With the means in place to counteract unwanted camcorder motion, all that's missing is a method to determine when it's happening. Camcorders have two common ways to do this as well, one relying on the CCD and the other on more complex mechanics. Sound familiar?
The simplest way to detect camcorder motion is to evaluate the output of the CCD itself. If the camcorder is shooting a subject that's not filling the whole viewfinder (which is usually the case), only a small portion of the screen will be moving at any given time. When the whole image moves, it's a pretty good bet that it is the camcorder itself, not the subject, that is in motion.
When the image stabilization circuit senses the whole image changing in the same direction, it assumes the camcorder is moving and will attempt to compensate. Usually, the camcorder will run algorithms that evaluate how fast and what direction the motion is occurring. These help the camcorder distinguish between desired movements and unwanted shake.
The other method to detect camcorder motion is to sense it directly. Motion sensors mounted in the body of the camcorder can detect camcorder motion on all three axes, and compensate accordingly. As with the optical image stabilization system, motion sensors are more complex and costly to implement than the simpler CCD system. Because the motion sensors are actually detecting the camcorder's movement, however, this approach is less prone to false readings.

What if I Want to Pan?
Though they have the almost magical ability to dampen unwanted camcorder shake, image stabilization schemes do have a dark side. The main drawback to image stabilization is its tendency to try to eliminate intentional camcorder motion. No stabilization system can tell the difference between an accidental bump to the left, and the beginning of a pan. After you've sustained the motion for a second or two, the system can be sure it's a desired motion.
This causes many stabilization systems to fight the first few degrees of a pan or tilt, and continue drifting a few degrees at the end. More than just unnerving, this can make a mess of your most calculated, professionally smooth pan. For this reason, many people disable their image stabilization when intentionally moving the camcorder.
You can also trick the systems that evaluate the CCD to determine if the camcorder is in motion when a large portion of the screen contains a moving subject. This could be a train pulling out of the station, a van driving by, or even just a person moving near the lens. One time, a camcorder tested by can Videomaker had an interesting reaction when we bounced a large cardboard box a few feet in front of the lens. With EIS engaged, the box stayed stationary on the screen while the rest of the world seemed to be moving up and down.
Finally, certain EIS systems can cause a noticeable drop in image resolution. This is because they're creating the image from a smaller block of pixels on the CCD, and enlarging them to fill the screen. A camcorder with a 280,000-pixel CCD, for example, may be using fewer than 250,000 pixels to create the image with EIS engaged.
The best solution is to load the CCD with extra pixels so engaging EIS has no visible effect on image quality. Several higher-priced camcorders use this approach, and the results are excellent--it's truly impossible to detect a drop in resolution with EIS engaged.

Steady As She Goes
For most people, the drawbacks of image stabilization pale compared to its many benefits. Playback tells the whole story--image stabilization has made the world of handheld video a much smoother place to be.