I hear people talking about vibration reduction giving you extra 'stops' on a lens, thus giving you extra shutter speed to play with.
How do people work this out into an amount of stops?
E.g. by using the VR on this lens it gives you about an extra 1/4 stop in shutter speed.
I don't know anyone who calculate those kind of thing, people just pull numbers out of their .. hemm .. I mean people produce approximate numbers based on their rich experience.
If you did want to calculate this the method would be:
Obviously the "slowest shutter speed that give you sharp hand-held images" depends on a lot of external factors (from camera holding technique to how tired you are at the moment) so that is why the numbers are mostly made up, sorry, I meant to say approximation based on experience.
First we should explain what a 'stop' (or more correctly, an 'f-stop', the ratio of the lens's focal length to the diameter of the entrance pupil.) is: it means a doubling or halving of the light gathered during an exposure by opening or closing your aperture. But more generally, the same exposure effect (purely in terms of the amount of light, ignoring other effects) can be achieved, via the infamous exposure triangle, by changing iso, aperture or time. Double one of these, and you increase exposure by a stop.
Now, if you test a feature like VR, you take test shots with VR off and find the point where they start to get blurry. Now you turn VR on and repeat. You'll may find that you achieve the same quality with an exposure time 4 times as long. This means you gained two f-stops. You can close your aperture by two stops, and still get a sharp shot, thanks to the longer time.
The "vibration reduction", "image stabilization", "optical stabilization" or whatever the manufacturer calls it is a function that uses small optical elements to counter the small erratic movements that the photographer introduces when handholding the camera.
When handholding a camera without this stabilizing function a rule of thumb is to not use a shutter speed slower than 1/[focal length in mm] seconds. Using a slower shutter speed is likely to induce blur from the movement of the camera during the exposure. A stabilized lens counteract this to a certain degree.
Assume that a lens with stabilization turn of gives sharp results hand held at a shutter speed of a 1/60th of a second. Turning it on could very well push that limit to 1/15th of a second. This allows you to gather 4 times as much light that can be used for other things (such as closing down the aperture). This is what people mean when they tell you that the vibration reduction is giving you the extra playroom.
Of course the number of fstops that you gain by turning on the stabilization is not calculated by rather a result of the photographers experience as it depends on the shooting techique and the resolution of the camera among other things.
