Thursday, February 5, 2009
March/April 2009: Helpline
Holding Steady: Presetting Exposure & Focus | Stable-Enabled
Q I’ve finally gotten a lens for my camera that offers image stabilization, but I was surprised to find that the lens has a switch on it to turn it off. I know my manual talks about turning it off when I have the lens on a tripod, but why?
Via the Internet
A Image stabilization is an option built in to many lenses and even some camera bodies. Manufacturers use a variety of terms to indicate a lens has stabilization. Canon uses “IS” for Image Stabilizer; Nikon uses “VR” for Vibration Reduction; Tamron uses “VC” for Vibration Compensation; and Sigma uses “OS” to indicate Optical Stabilization.
Acronyms aside, the technology works to remove image blur caused by camera movement. Motion sensors in the lens sense any camera movement. This movement is converted into an error voltage that’s then transmitted to an optical section of the lens used to compensate for motion. The optical correction module is made up of one or more optical elements contained in special moveable mounts. Tiny correction voltages are applied to the mounts to counteract or reverse the camera movement.
Other camera manufacturers, like Olympus, Pentax and Sony, accomplish the stabilization in the camera body rather than the lens. In this situation, the motion sensors are in the body of the camera, and the correction signal is sent to the image-sensor mount. The mount then moves the whole image sensor to compensate for camera motion.
All of this technology helps keep your images sharp and allows you to handhold your camera at slower shutter speeds. Some manufacturers say that you can handhold at shutter speeds up to two to four stops slower than you’d normally shoot. But that depends on how good you are at holding your camera steady.
If you’ve never used a stabilization system, it may take a bit to get used to it. Some are “on” all the time; others only engage when you press the shutter release halfway. If you’re using a lens-based stabilization system, when you look through the optical viewfinder, you can see the system engage—the image shifts a little bit as the correction is applied. (Users of in-body stabilization would see the stabilization in action when viewing on the LCD or through an electronic viewfinder.)
As you’ve noticed on your lens, there’s an option to turn off the correction. With in-body correction, you’ll find either a button on the camera or a menu option to disable the correction.
There are several reasons why you might want to turn off stabilization. For example, when your camera is mounted on a tripod, the sensors may interpret sensor noise as something that needs to be corrected. In this case, if you leave the feature on, it will degrade your image. When you correct something that doesn’t need correction, it looks like you moved the camera.
If you look at the image at the top of my column, you can see a photo I took in Cologne, Germany, one early, foggy morning. Because of the long exposure, I mounted the camera on a tripod and used both a remote shutter release and mirror lockup to reduce any camera vibration. I took the picture first with image stabilization on and then with it off.
If you pay attention to the lights in the scene, you can see that the image with the most blur is the one with stabilization turned on. The light trails show the error correction that has been applied to the lens elements. It’s almost like the stabilization is operating in reverse.
Although manufacturers now build their systems to be sensitive enough that you must turn off the correction when using a tripod, you really should perform some tests to see how...
Although manufacturers now build their systems to be sensitive enough that you must turn off the correction when using a tripod, you really should perform some tests to see how your camera- and-lens combination works.
Generally, there’s also an option to turn the image stabilization off in one axis. If you think about how your camera can move normally, the compensation corrects for up-and-down and side-to-side movement. If you pan with your camera to capture an image—such as a horse racing down a track—there will be obvious camera movement that the motion sensors will pick up. Since the correction can correct only so much error, it would be nice if you could set the stabilization so that it works only in one direction, namely in the vertical direction. That’s why there’s a “panning” mode for stabilization. This mode turns off the correction for side-to-side movement, but still allows the system to correct for any up-and-down motion.
Finally, another reason to turn off vibration reduction relates to power. The correction system involves sensors and devices that move. That all translates to using energy. When you use image stabilization, your camera won’t be as energy-efficient. If you try to eke out the most power from your battery and you can work with faster shutter speeds or a tripod, you can turn off correction.
You can find shake-reduction systems in all sorts of cameras, not just those with interchangeable lenses. Advanced compact cameras operate in much the same way as their SLR counterparts, with the exception of some models that use high ISO to achieve faster shutter speeds. These cameras are sometimes labeled “digital image stabilization” and don’t use any type of sensor or optical correction.
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