Shutter speed controls two things at once: how much light reaches the sensor, and how much movement is recorded while the shutter is open. Most exposure advice concentrates on the first. This page deals with the second, because motion blur is geometry — how fast the subject's image crosses the sensor — and geometry can be calculated rather than guessed.
This shutter speed calculator from Arb Digital works out the exposure time that keeps a moving subject's image within a blur tolerance you set, and reports the two conventional limits that constrain the same shot from other directions: the reciprocal rule for camera shake and the 500 rule for star trailing. It is a companion to our exposure value calculator, which balances brightness across aperture, shutter and ISO but says nothing about movement.
What This Shutter Speed Calculator Does
Enter the subject's speed, how far away it is, the angle at which it is travelling relative to the lens axis, your focal length, the sensor's crop factor and pixel width, and how much blur you are prepared to accept in pixels. The tool converts that into the exposure time at which the subject's image moves exactly your tolerance across the sensor, then snaps it to the nearest standard shutter speed that is at least that fast.
Alongside it you get the reciprocal-rule handheld limit for your effective focal length, the same limit extended by the stabilisation you have available, the 500-rule star-trail limit, and how many pixels of subject blur the shutter speed you actually planned to use would produce. Those four together usually settle the question of whether a shot is possible before you take it.
How to Use It
- Estimate the subject's speed honestly. A person walks at roughly 5 km/h, a jogger at 10, a cyclist at 25 and traffic in a town at 30 to 50. Errors here scale the answer directly.
- Measure distance, not focus setting. Blur scales inversely with distance, so a subject at ten metres blurs five times as fast across the sensor as the same subject at fifty.
- Set the angle. Ninety degrees is a subject crossing the frame and is the worst case; reduce it for a subject moving diagonally.
- Set a blur tolerance you actually need. One pixel is for pixel-level scrutiny. Three is generous for a print or a screen. Ten is fine for a small web image.
- Check both limits. If the handheld limit is slower than the freeze speed, the shot is shake-limited and stabilisation helps. If the freeze speed is slower, stabilisation does nothing at all.
How the Freeze Speed Is Calculated
For a subject far enough away that the lens is focused near infinity, the angular rate at which it crosses the field of view is its transverse speed divided by its distance. The image on the sensor moves at that angular rate multiplied by the focal length. Divide the blur you will tolerate, expressed in millimetres on the sensor, by that image speed, and you have the exposure time.
Work the defaults. Thirty kilometres per hour is 8.333 m/s, and at 90° all of it is transverse. At 25 m the angular rate is 8.333 ÷ 25 = 0.3333 radians per second. With a 100 mm lens the image moves at 100 × 0.3333 = 33.33 mm per second. A 1.5× crop sensor is 24 mm wide, so 6,000 pixels across gives a pixel pitch of 0.004 mm, and a three-pixel tolerance is 0.012 mm. The exposure time is 0.012 ÷ 33.33 = 0.00036 s, or 1/2,778 s, which snaps to a standard 1/3,200 s.
Two things follow from the formula that are worth internalising. Blur scales with focal length, so doubling the focal length halves the shutter time you can use on the same subject. And blur scales inversely with distance, so backing off is a legitimate technical solution and not just a compositional one.
The Reciprocal Rule and the Crop-Factor Convention
The reciprocal rule says the slowest handheld shutter speed is one over the focal length in millimetres: 1/100 s for a 100 mm lens. The convention on a cropped sensor is to use the effective focal length, which is the actual focal length multiplied by the crop factor. A 100 mm lens on a 1.5× body is treated as 150 mm, giving 1/150 s, which snaps to a standard 1/160 s.
The rule is a rule of thumb from 35 mm film practice, not a physical law. It originated when a sharp result meant a modest print, and modern sensors resolve far more detail, so many photographers treat it as one to two stops optimistic for critical work. It also says nothing about the photographer: a braced stance against a wall buys more than most stabilisation systems.
Stabilisation is quoted in stops, and each stop doubles the usable exposure time. Three stops takes 1/160 s to 1/20 s. It is worth being precise about what that buys, though: stabilisation counteracts camera movement only. It does absolutely nothing for a moving subject. A stabilised long lens will happily deliver a rock-steady, thoroughly blurred photograph of a running dog.
The 500 Rule, and Why It Is a Convention
For a static camera photographing stars, the sky rotates and the stars trail. The 500 rule divides 500 by the effective focal length to give the longest exposure in seconds before trailing becomes obvious: 500 ÷ 150 = 3.3 s for the defaults. Some photographers use 400 or 300 for a stricter result, and the fact that three different numbers are all in common use is the clearest possible signal that this is a convention rather than a derivation.
It also ignores three real variables. Stars near the celestial equator trail fastest and stars near the pole barely move, so declination matters. Pixel pitch matters, because a denser sensor records a trail that a coarser one would not resolve. And output size matters, because a trail invisible in a web image is obvious in a large print. Treat the number as a starting point, take a test frame, and inspect it at full magnification.
When the Formula Stops Applying
Set the angle to zero and the transverse component vanishes, so the formula returns an unbounded exposure time. That is mathematically correct and practically useless: a subject moving straight at the camera still changes size, still moves through the depth of field, and will still be ruined by a slow shutter. What limits that shot is focus tracking and subject-distance change, not transverse blur, and this page does not model either. The tool says so rather than printing a fantasy number.
The formula also assumes the subject is far enough away that the image distance is close to the focal length. In close-up and macro work that assumption fails, magnification rises steeply, and a subject moving a millimetre can move many pixels. If you are working at high magnification, treat the freeze speed here as an underestimate and test. For the focus side of that problem, our depth of field calculator and hyperfocal distance calculator are the right tools.
Deliberate Blur Is a Choice, Not a Failure
Everything above assumes you want the subject sharp. Often you do not. A panned shot follows the subject so that it is sharp while the background streaks, which requires a shutter speed slow enough to record background movement — the opposite of the calculation on this page. Waterfalls, light trails and crowd blur all work the same way, by choosing an exposure time deliberately longer than the freeze speed.
The useful move is to calculate the freeze speed anyway and then deliberately step away from it by a known number of stops, so the effect is repeatable. Going from 1/3,200 s to 1/60 s is a little over five stops, which is a large brightness change that must be paid for with aperture, ISO or a neutral density filter. Our exposure value calculator handles that trade directly, and the crop factor calculator covers how the same lens behaves across sensor formats.
Arb Digital's content and web design teams handle the whole chain from shooting brief to the compressed, correctly sized asset that loads fast on your site.
Content Marketing Services Web Design ServicesCommon Mistakes to Avoid
- Expecting stabilisation to freeze a subject — it corrects camera shake only and does nothing about movement in the scene.
- Applying the reciprocal rule to the marked focal length on a cropped sensor, which understates the required speed by the crop factor.
- Ignoring distance — the same subject at half the distance needs twice the shutter speed at the same focal length.
- Using a one-pixel blur tolerance for everything, which demands enormous shutter speeds for output that will never be inspected that closely.
- Treating the 500 rule as exact, when declination, pixel pitch and print size all move the real limit.
Related Free Tools From Arb Digital
Balance brightness with the exposure value calculator, compare formats with the crop factor calculator, and handle focus with the depth of field calculator and the hyperfocal distance calculator. For output, the aspect ratio calculator covers cropping and sizing. The free online tools hub lists the rest.
Frequently Asked Questions
There is no single answer, because it depends on the subject's transverse speed, its distance, the focal length and how much blur you accept. The calculation is blur tolerance in millimetres divided by focal length times speed over distance.
It is a rule of thumb that the slowest safe handheld shutter speed is one over the effective focal length in millimetres. On a cropped sensor the convention is to multiply the marked focal length by the crop factor first.
For camera shake, yes: each quoted stop of stabilisation doubles the usable exposure time. For subject movement, no. Stabilisation corrects movement of the camera and has no effect on movement in the scene.
Divide 500 by the effective focal length to get the longest exposure in seconds before star trailing becomes obvious. It is a convention rather than a derivation, and 400 or 300 are also in common use for stricter results.
Blur depends on angular rate, which is speed divided by distance. A subject twice as far away crosses the frame at half the angular rate, so the same tolerance allows twice the exposure time.
The transverse component of its motion falls to zero and this formula stops applying. What limits the shot then is focus tracking and the change in subject distance, which this page does not model.
It depends on output. One pixel is a pixel-peeping standard, around three is a sensible working tolerance for prints and full-screen viewing, and ten or more is unnoticeable in a small web image.
For the underlying optics, Stanford's photography course materials cover both halves of this problem directly: Variables that affect exposure explains how exposure time produces motion blur and camera shake, and Depth of field covers the circle of confusion that sets what counts as sharp in the first place.
Figures produced by this tool are photographic planning estimates based on the transverse-motion geometry and conventions described above. Real results depend on subject behaviour, focus accuracy, lens and sensor characteristics and how the image will be viewed, so take a test frame and inspect it before relying on any single number.
