Image Stabilization (IS) is one of the features a lens might or might not have. When available (and turned on) on your lens, it will keep image blur due to camera shake to a minimum, and it will enable you to shoot with slower shutter speeds while maintaining sharp focus.
It's important to keep in mind that image stabilization will not solve blurry image problems if the motion is caused by your subject moving rather than you handholding the camera, or by extreme camera movement at and during exposure.
Image stabilization can often permit the use of shutter speeds 2-4 stops slower (exposures 4-16 times longer) than otherwise possible, and sometimes even more.
To calculate the slowest shutter speed that is possible for a handheld camera with no noticeable blurring of the image, it needs to be 1/lens focal length you're shooting with.
This means that if you're shooting with a 50mm lens, your minimum shutter speed should be no longer than 1/50, for example 1/50, 1/60, 1/125, 1/250...etc to avoid image blur.
Likewise, if you're shooting with a 70-200mm zoom lens at let's say 125mm focal length, your minimum shutter speed should be no longer 1/125, for example 1/125, 1/250, 1/500... etc. But if your lens offers image stabilization, you would be able to take the slowest shutter speed possible down an extra 2-4 stops or even more. This means you would be able to shoot at 1/15 or 1/8 or even slower, with no significant image blurring. This is of course assuming you have a 35mm format camera.
If you're not using a 35mm format, you need to note that the crop factor will affect the slowest shutter speed possible at a given focal length.
The crop factor, also known as the focal length multiplier (FLM), is the ratio of the dimensions of a camera's image plane compared to a reference format. Most often, this term is applied to digital cameras, relative to 35mm film format as a reference. In the case of digital cameras, the image plane would be a digital sensor.
Taking that into account, let's assume you're shooting with a 1.6x crop factor image plane at 50mm focal length. Your actual focal length would then be 50 X 1.6 = 80mm rather than 50mm. So the slowest possible shutter speed that will not result in image blurring while handholding your camera would be 1/80s or faster.
Image stabilization names differ from one manufacturer to another, but they all serve the same purpose and do the same job:
Canon
IS (image stabilization)
Nikon
VR (vibration reduction)
Sony Cyber-Shot
Optical SteadyShot
Sony
SSS (Super Steady Shot)
Panasonic / Leica
MegaOIS
Sigma
OS (Optical Stabilization)
Tamron
VC (Vibration Compensation)
Pentax
SR (Shake Reduction)
It's important to keep in mind though that image stabilization is not an alternative for a tripod, and that its preferable to keep it off when it's not needed (when you're already shooting at high enough shutter speeds or when it's setup on a tripod). Doing so will also lengthen your battery life since IS draws more battery power.
Most manufacturers actually do suggest that the IS feature of a lens be turned off when the lens is mounted on a tripod as it can cause unexpected results, and is generally unnecessary.
Many modern image stabilization lenses are able to detect when and when not your camera is being mounted on a tripod, due to significantly low vibration readings. A great example of those are canon's newest IS lenses.
When extremely low vibration is detected, those lenses are programmed to automatically disable image stabilization on their own.
Image stabilized lenses also have disadvantages. One main problem would be the high price tag that goes along with them. Image stabilized lenses can also yield poor bokeh results due to light shifting from its true optical path while passing through the lens, something you might be concerned about.
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