The old guide is one over the focal length, which is arithmetic rather than a measurement: at 200 mm, about 1/200 s. It says nothing about your hands, your stance, or the subject — only about the lens.
The guide is one number divided by another, and its usefulness comes from being roughly right rather than from being measured.
The arithmetic
Take the focal length in millimetres and use one over it as a shutter speed: 50 mm gives about 1/50 s, 300 mm about 1/300 s. Longer lenses magnify angular movement, so the same tremor displaces the image further.
What it leaves out
The subject. A still building at 1/50 s is sharp; a walking child is not, at any focal length. Subject movement is answered by shutter speed alone, which means by light.
Your stance. Braced against a wall with elbows in is several stops better than one hand at arm’s length, and nothing publishes that.
The format. On the smaller of the two formats here, the same engraved focal length frames tighter, so the shake shows sooner.
What the catalogue publishes on the other side of it
The aperture pair — f/3.5-6.3 on a typical kit zoom — which decides how much shutter speed is available at all. At the long end, two and a half stops are already gone before any rule is applied, and that is the figure worth reading first.
Where this page and the maker’s own manual disagree, the manual decides. Every focal length, aperture, guide number, stop of stabilisation, load rating and sealing claim on this page is reported from the manufacturer’s own documentation, with its unit written as published, and named as such — none of it is measured here. An engraved figure is reported as the maker engraves it, and is never restated in another convention, because the two are not the same number. And for any figure quoted in stops: a stabilisation rating is a laboratory result obtained under a named protocol, not a promise about handheld work. Where a maker names the protocol and the focal length it was measured at, both are reported here beside the number. Where none is named, that absence is reported too, and no figure is inferred in its place.
What people ask before they commit
Does the rule change on a smaller sensor?
The framing does, so the shake shows more for the same engraved focal length — a 200 mm lens on the smaller format frames as a longer lens would on the larger. Multiplying the engraved figure by the ratio of frame widths before applying the rule is the usual correction, and it is a calculation each time.
How many stops does stabilisation add to this?
Whatever the maker publishes, measured to a protocol it names — and in this catalogue **no title names a protocol at all**. A bare stop count is comparable only with another bare stop count. Treat it as a margin rather than as a licence, because the rule it modifies was never about your hands either.
Last reviewed 12 September 2026