so does the reciprocal guide apply to actual focal length or equivalent focal lenght? :dunno:
i'm pretty sure shake is more apparent when you're zoomed in closer.
The issue of whether to use actual focal length or equivalent focal length has been debated many times. Most of the websites will prefer to state the equivalent focal length.
There are arguments for and against using equivalent focal length instead of actual focal length. Every one of them has its seemingly valid theoretical arguments (I shall not get into the details of that).
However, practical experience is otherwise. Take for example, the Olympus SP550UZ which, at its max. zoom, is 507mm on 35mm format equivalent but people are able to shoot at 1/15-1/25 handheld with image stabiliser on. Is the image stabiliser good for 5 stops? No. The explanation is that the actual focal length is only 84mm and the image stabiliser works for about 2-3 stops. For DSLR, the practical experience is similar for different "crop factor" sensor sizes.
Look at the lowest shutter speed those compact digicams are capable of shooting handheld at and you will see why the acutal focal length in the reciprocal guide is more practical and valid than the equivalent focal length. In the past, without image stabiliser, compact digicams can be handheld at 1/15-1/30 at max 3x optical zoom easily even at 105mm equivalent because the actual focal length is no more than 20mm.
As long as the shutter speed is fast enough, the light signal is prevented from spreading itself across too large a sensor area to create a blur outline.
Zoom in = longer focal length means that, given the same amount of handshake, the light ray will shake more as it has to travel a longer distance from optical centre to the sensor/image plane. So a faster shutter speed is required to prevent the light signals from spreading across too many pixel pitches to create a blur picture.
So I would say : If you have average steady hands, use 1/actual focal length. If you have shaky hands, use 1/equivalent focal length. If you have very shaky hands, then use 1/equivalent focal length + 1 stop faster shutter speed.