How slow can you go?
What an Image Stabilizer’s ‘stop’ rating actually means
The rule of thumb for sharp handheld shots is to ensure the shutter speed is at least equivalent to (ideally faster than) the focal length of the lens. So with a 60mm lens, the shutter speed should be at least 1/60 sec to avoid the effects of camera shake. But if you’re shooting in low light, or you want to use a smaller aperture, the shutter speed may be slower. You can, increase the ISO to achieve a faster shutter speed, but you may not be able (or want) to push it. This is where an IS lens can save you.
Canon’s image stabilization systems, whether they’re in-lens or in-camera, are rated in ‘stops’. A stop is a measure of exposure. When a lens says it offers four stops of image stabilization, this means the camera shake reduction is equivalent to using a shutter speed that’s four times faster. So, with IS switched on, a shutter speed of 1/25 sec could potentially give an image that’s as sharp as one taken at 1/200 sec. This is because each ‘stop’ is equivalent to a shutter speed that’s twice as fast: 1/25 sec – 1/50 sec – 1/100 sec
– 1/200 sec.
I say ‘potentially’ because your mileage may vary when it comes handheld shots. If your technique isn’t so good, or the lens is bulky, or you’re shooting in windy conditions, then you might not be able to get sharp results at the number of stops indicated. Canon measures the effective performance of the IS system using a vibrating machine, rather than actual handheld shots, so your experience of using the lens may be different.