What it's for
The "reciprocal rule" is a classic photography rule of thumb that estimates the slowest shutter speed that still produces sharp handheld shots, no tripod, accounting for natural hand shake. This calculator applies the rule and, if your lens or camera has image stabilization, calculates how much slower that speed can safely go.
Formula used
The classic rule is safe_speed = 1 / effective_focal_length (in seconds), where the effective focal length already accounts for the sensor's crop factor: effective_focal_length = focal_length × crop_factor. For example, a 50mm lens suggests a minimum speed of 1/50s. With image stabilization, the safe speed can be reduced: speed_with_stabilization = safe_speed / 2^stabilization_stops, where the stabilization stops are the value claimed by the lens or camera manufacturer (IS, VR or OSS, depending on the brand).
Why multiply by the crop factor
Smaller sensors enlarge the final image more for the same framing, which also magnifies the visual effect of hand shake — that's why the effective focal length (which already includes this factor) is the correct base for the rule, not just the lens's nominal focal length.
Important limitations
This is a starting-point rule of thumb, not a guarantee — the real result depends a lot on the photographer's technique (breathing, support, muscle tension), how much a person's hands naturally shake, and any movement in the subject itself (which image stabilization does not compensate for). Many photographers prefer extra safety margin, especially for critical shots.
Frequently asked questions
What do "IS", "VR" and "OSS" mean?
They're different brand names for the same image stabilization technology: IS (Image Stabilization, used by Canon), VR (Vibration Reduction, Nikon) and OSS (Optical SteadyShot, Sony). The claimed stops vary by specific lens/camera.
Does image stabilization compensate for subject movement?
No — it only compensates for the photographer's hand shake. If the subject (a person, an animal, an object) is moving, you still need a fast enough shutter speed to freeze that movement, regardless of stabilization.
Why does the rule use crop factor instead of just the lens's focal length?
Because what matters for visible shake in the photo is the effective (full-frame-equivalent) focal length, since smaller sensors magnify the effect of any hand shake more in the final image.