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Guide to Image Stabilisation

Safe shooting
There’s a rule of thumb that links the minimum ‘safe’ shutter speed to the focal length in use. The speed is one divided by the focal length, or the reciprocal of the focal length.

Things have got more complicated, though, with the arrival of the digital camera and different sensor sizes. For example, a digital compact might have a 7-21mm zoom, which suggests you should be able to shoot at a seventh of a second at the wide-angle end of the range and 1/21sec at the long end. This isn’t the case. What you need to do is work out the speeds using the equivalent focal length in 35mm film camera terms. In this case, our digital compact has an equivalent focal range of 35-105mm, which gives us more realistic ‘safe’ speeds of 1/35sec at the wide-angle end and 1/125sec at the long end (rounded to the nearest shutter speed).

Digital SLRs generally have a focal factor of 1.6x the focal length on the lens, so that an 18-70mm zoom lens, for example, is approximately equivalent to a 28-110mm (35mm). You can work out safe shutter speeds from this.

Our chart (below) gives you an idea of where this danger area is for any given equivalent focal length. We’ve done the same for image-stabilised cameras, allowing another two stops of shake-free shooting to demonstrate the kind of advantage they have.

In practice, of course, there’s no sharp cut-off point. Our chart should include broad bands of ‘marginal’ speeds rather than narrow lines indicating single values, though attempting to reproduce that in a diagram would quickly get confusing. You can sometimes get sharp shots at half the safe speed or blurred shots at twice that speed. It all depends on the conditions, the camera design and how good you are at holding a camera steady. It’s better to think of a ‘danger’ area rather than a single shutter speed limit.

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