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Why moving subjects blur, and how much is too much
A photograph freezes a slice of time, but a moving subject keeps moving while the shutter is open. During that short exposure, the subject’s image slides across the sensor, and that slide becomes blur. The faster the subject, the closer it is, and the longer the shutter stays open, the longer the slide, and the softer the subject looks. A runner far away may look sharp at 1/500 of a second, while the same runner sprinting past the camera may need 1/4000 to look equally sharp. This calculator turns those facts into a number: how many pixels of blur your settings will produce, and how fast your shutter must be to keep blur under a single pixel.
Four things decide the blur. Subject speed is the first: a car at 100 kilometres per hour crosses far more of the frame during an exposure than a walking person. Distance is the second: a subject close to the lens appears to move faster across the frame than the same subject far away, because the angle it sweeps is larger. Focal length is the third: a long telephoto lens magnifies the subject, and it magnifies its movement too. Shutter speed is the fourth, and it is the one you control most easily. Halving the exposure time halves the blur, which is why sports photographers reach for very fast shutters when the action comes close.
How to use this motion blur calculator
Enter the subject’s speed in kilometres per hour, its distance from the camera in metres, your focal length in millimetres, and the shutter speed you plan to use. Then add your sensor width and image width in pixels, so the tool can turn the blur on the sensor into pixels in your file. A full frame sensor is 36 millimetres wide, and a typical high resolution file is 6000 pixels wide, but you should use your own camera’s numbers for an accurate result. The tool shows the blur in pixels, the blur as a percentage of the frame width, and the shutter speed that would freeze the subject to under one pixel of blur.
As a worked example, take a cyclist at 36 kilometres per hour, 10 metres away, shot with a 50 millimetre lens at 1/1000 of a second on a full frame camera with a 6000 pixel wide file. The blur comes out at about 8.3 pixels, which is small in a 6000 pixel file but clearly visible if you zoom in or print large. To freeze that cyclist to under one pixel, you would need a shutter faster than about 1/8300 of a second, which most cameras cannot reach. That does not mean the photo is ruined. It means you should decide what sharp means for your purpose: a portrait printed small may look perfect at 8 pixels of blur, while a large exhibition print of the same file may not.
Freezing motion versus showing it on purpose
Not every photo needs zero blur. A completely frozen athlete can look pasted onto the background, while a small amount of blur in the hands, feet, or wheels tells the viewer that something is moving. Wildlife photographers often accept a little blur in wingtips while keeping the eye sharp, and street photographers often let walking legs blur slightly while faces stay crisp. The skill is choosing the blur, not eliminating it by accident. Use the calculator to find the shutter speed for the blur you want, not just the fastest speed your camera allows, and check the result at full size on a screen before you trust it.
Distance is your secret weapon when light is short. If you cannot reach a fast enough shutter speed, step back or use a shorter lens, because both changes shrink the subject’s apparent speed across the frame. Doubling the distance halves the blur, just as doubling the shutter speed does. Moving so the subject travels towards you rather than across the frame helps even more, because motion towards the camera slides sideways very little, even though the subject grows larger in the frame. Combine these choices: a slightly longer distance, a subject moving at an angle, and the fastest shutter your light allows, and you will bring home far more sharp action shots than shutter speed alone would give you.
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