Shutter Speed to Freeze Motion Calculator

Freezing motion is a question about the image plane, not about the subject. For a subject much farther away than the focal length, the magnification is approximately the focal length divided by the subject distance, so a subject crossing the frame at some speed produces an image that sweeps across the sensor at that speed multiplied by the same ratio. The required exposure time is then the blur you are willing to accept, expressed as a distance on the sensor, divided by that image-plane rate. Turning a pixel tolerance into millimetres needs the pixel pitch, and the 0.005mm used here is representative of current cameras, since a 24 megapixel full-frame sensor has a pitch near 5.9 micrometres and a 45 megapixel one near 4.4. The relationships are all linear and worth internalising: doubling the focal length or halving the distance each double the required shutter speed. Field practice lands around 1/500 second for pedestrians, 1/1000 to 1/2000 for field sports, and 1/2000 or faster for birds in flight, while the reciprocal rule, one over the effective focal length, addresses a different problem entirely, namely camera shake rather than subject motion. The model is deliberately simple and its assumptions are where it breaks. It treats the subject as moving perpendicular to the optical axis, so a subject approaching at an angle produces less image-plane motion by roughly the cosine of that angle, and a subject moving straight toward the camera almost none. It treats the subject as rigid, which understates the problem badly for limbs, wingtips and club heads that rotate at several times the speed of the body they belong to, and it ignores panning, which cancels the subject rate while deliberately smearing the background. It also ignores the mechanical ceiling of the camera, since most bodies stop at 1/8000 second, which is why the calculator can demand exposures that no shutter will deliver and the honest response is to accept more blur, shoot from farther away, or use a shorter lens. Every halving of exposure time also costs a stop, paid in aperture or in ISO noise.

Stopping motion was the central technical ambition of nineteenth century photography, because early emulsions were far too slow to record anything that moved. The decisive demonstration came in 1878, when Eadweard Muybridge, working at Leland Stanford's Palo Alto stock farm, arrayed a bank of cameras with mechanically tripped shutters along a track and produced sequential images of the horse Sallie Gardner, settling the long-running argument about whether a trotting horse ever has all four feet off the ground. Etienne-Jules Marey pursued the same problem in France through the 1880s with chronophotography, recording successive phases of motion on a single plate to study animal and human locomotion. The hardware that made short exposures routine was the focal-plane shutter, a slit travelling close to the sensitised surface, which Ottomar Anschutz used to reach around a thousandth of a second in the 1880s. The next order of magnitude came from light rather than mechanics. Harold Edgerton, an electrical engineering professor at MIT, developed the electronic stroboscope from the early 1930s and drove flash durations down to microseconds, producing the images that made the technique famous, including the milk drop coronet and the bullet passing through an apple. Because the flash rather than the shutter defined the exposure, motion could be frozen at speeds no mechanical curtain could approach. The familiar shutter scale itself was rationalised in the middle of the twentieth century, when international standardisation replaced older irregular sequences such as 1/25, 1/50 and 1/100 with the doubling series of 1/125, 1/250 and 1/500 that makes each step exactly one stop and allows shutter, aperture and sensitivity to be traded against one another arithmetically.