What is the reciprocal rule?

It's a guideline that the slowest hand-held shutter speed for a sharp photo is roughly one over the focal length in seconds. At 50 mm on full-frame that's about 1/50 s; at 200 mm it's about 1/200 s, because longer lenses magnify shake more.

Why does sensor crop change the result?

Crop sensors magnify the field of view, which also magnifies any camera shake. The rule uses the 35mm-equivalent focal length, so a 50 mm lens on an APS-C body behaves like a 75–80 mm lens and needs a faster shutter than 1/50 s.

What does the strict (1/2×focal) option do?

High-resolution sensors resolve fine detail, so they reveal shake the classic rule would miss. The strict variant doubles the required shutter speed (one over twice the focal length) for a safer margin on demanding cameras.

How does image stabilisation change things?

Each stop of stabilisation lets you halve the shutter speed while keeping the same sharpness. Four stops of IBIS plus lens IS, for example, turns a 1/50 s limit into roughly 1/3 s — though real-world gains are often a stop or two less than rated.

Does this account for a moving subject?

No. The reciprocal rule only addresses blur from your own hand movement. A walking person, sports or wildlife will blur at these speeds regardless of stabilisation; freeze subject motion with a much faster shutter, often 1/500 s or higher.

Camera Motion Blur & Shutter Speed Threshold Calculator

A sharp hand-held photo depends on a shutter fast enough to outrun your own movement. The classic reciprocal rule gives that threshold from focal length — this tool refines it for your sensor size and stabilisation so you know the slowest shutter you can safely use.

How it works

The rule starts from the focal length, but on anything other than full-frame you must use the 35mm-equivalent focal length, because crop sensors magnify shake along with the image:

effective_focal = focal_length × crop_factor
slowest_shutter = 1 / effective_focal      (seconds)

A 50 mm lens on an APS-C body (1.5× crop) behaves like 75 mm, so the limit tightens from 1/50 s to about 1/75 s.

Two adjustments refine this:

Strict / high-resolution mode doubles the requirement (1 / (2 × effective_focal)) because high-megapixel sensors reveal shake the original rule tolerated.
Stabilisation buys back time. Each stop lets you halve the shutter speed, so the usable shutter time is multiplied by 2^stops. Four stops turns a 1/60 s limit into roughly 1/4 s.

Tips and notes

Treat the result as the slowest you should go — faster is always safer for camera shake.
Rated stabilisation is optimistic; in practice subtract a stop or two from the headline figure.
This only fights your movement. Freeze a moving subject with a much faster shutter regardless of focal length or stabilisation.
Below the limit, brace against a wall, tuck your elbows in, use a tripod, or shoot a burst and keep the sharpest frame.