Shutter Speed Calculator: Equivalent Exposures and Motion Blur
When you change the aperture on a camera, the amount of light reaching the sensor changes and you must adjust your shutter speed (or ISO) to compensate and maintain the same exposure. This calculator finds the new shutter speed required after an aperture change, using the reciprocity relationship: new shutter time = old shutter time multiplied by the new f-number squared divided by the old f-number squared. It also calculates how many stops of difference exist between the two apertures. The second function uses the 1/focal-length handheld rule: the slowest safe shutter speed for a sharp shot without a tripod is approximately 1 divided by the focal length in millimetres. At 100 mm that is 1/100 s; at 300 mm it is 1/300 s. This rule is a guideline, not a guarantee. Subject movement, camera stabilisation, and shooting technique all affect sharpness. Modern image-stabilised lenses and bodies can safely shoot 2 to 5 stops below the rule, but the rule remains a useful starting point, especially for lenses without stabilisation. Enter your current shutter speed, current aperture, new aperture and focal length to see the required new shutter speed, the stops difference and the safe handheld threshold.
New shutter speed: -- | Stops change: --
How equivalent shutter speed is calculated
When you change aperture by a number of stops, the shutter speed must change by the same number of stops in the opposite direction to maintain the same exposure. The exact relationship comes from the definition of EV:
new_t = old_t × (new_N^2 / old_N^2)
stops_change = log2(new_N^2 / old_N^2) = 2 × log2(new_N / old_N)
safe_handheld = 1 / focal_length_mm (in seconds)
Where old_t is the current shutter time in seconds, old_N is the current f-number, and new_N is the new f-number. The stops_change value is positive when moving to a narrower aperture (higher f-number) and negative when widening.
Worked example
Current: 1/125 s at f/4. New aperture: f/8. Focal length: 50 mm.
- old_t = 1/125 = 0.008 s. old_N = 4. new_N = 8.
- new_N^2 / old_N^2 = 64 / 16 = 4
- new_t = 0.008 × 4 = 0.032 s (approximately 1/30 s)
- Stops change = log2(4) = +2.00 stops (narrower by 2 stops)
- Safe handheld = 1 / 50 = 0.020 s (1/50 s)
- New shutter 1/30 s is slower than 1/50 s: use a tripod or raise ISO
Shutter speed reference: common uses
| Shutter speed | Typical use | Motion result |
|---|---|---|
| 1/2000 s and faster | Sport, birds in flight, splashing water | Freezes fast motion completely |
| 1/500 s to 1/1000 s | Running, cycling, moving vehicles | Freezes most motion |
| 1/125 s to 1/500 s | Portraits, street, general handheld | Sharp for normal movement |
| 1/30 s to 1/125 s | Stationary subjects, low light handheld | Risk of camera shake below 1/focal-length |
| 1/15 s to 1/2 s | Panning, creative blur, twilight | Background or subject blur |
| 1 s and slower | Starry sky, light trails, waterfalls | Significant motion blur; tripod required |
The 1/focal-length handheld rule
The 1/focal-length rule is a guideline stating that the slowest safe handheld shutter speed is 1 divided by the focal length in millimetres. At 50 mm, that is 1/50 s; at 200 mm, it is 1/200 s. The rule exists because longer focal lengths magnify camera shake more than wide angles.
On a crop-sensor camera, the effective field of view is narrower, so you should apply the crop factor to get the equivalent focal length before applying the rule. An APS-C body with a 1.5x crop factor turns a 100 mm lens into a 150 mm equivalent, so the safe minimum becomes 1/150 s, not 1/100 s.
Image stabilisation (optical or in-body) extends safe handheld shooting by 2 to 5 stops depending on the system. A lens rated for 4 stops of stabilisation at 200 mm could, in theory, allow shots as slow as 1/12 s. In practice, subject movement (not just camera shake) often limits how slow you can go before blur appears, regardless of stabilisation.
Shutter speed calculator: frequently asked questions
What is the reciprocity principle in photography?
Reciprocity means that any combination of aperture and shutter speed that admits the same total amount of light produces the same exposure. If you open your aperture by one stop (e.g. f/8 to f/5.6), you double the light, so you must halve the shutter speed (e.g. 1/125 s to 1/250 s) to maintain the same exposure. The relationship is: new shutter time = old shutter time x (new f-number squared / old f-number squared). This calculator does that arithmetic for you.
What is the 1/focal-length rule for handheld shooting?
The 1/focal-length rule states that the minimum safe shutter speed for a sharp, handheld shot is 1 divided by the focal length in millimetres. For a 50 mm lens, the minimum is 1/50 s; for a 200 mm lens, it is 1/200 s. Longer lenses magnify camera shake more, so they require faster shutter speeds. On a crop-sensor camera, multiply the focal length by the crop factor (e.g. 1.5x for APS-C) before applying the rule: a 200 mm lens on an APS-C body behaves like 300 mm, so the safe minimum is 1/300 s.
How many stops are there between f/2.8 and f/8?
There are three full stops between f/2.8 and f/8. The full-stop f-number sequence is: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Counting from f/2.8 to f/8: f/2.8 to f/4 (one stop), f/4 to f/5.6 (two stops), f/5.6 to f/8 (three stops). Because each stop halves the light, going from f/2.8 to f/8 reduces the light by a factor of 8 (2^3), requiring 8 times the shutter duration to maintain exposure.
What causes motion blur in photographs?
Motion blur occurs when a subject or the camera moves while the shutter is open. The longer the shutter is open, the more movement is recorded as blur. Fast-moving subjects (sport, wildlife) require fast shutter speeds (1/500 s or faster) to freeze motion. Slow shutter speeds (1/30 s and below) allow blur, which can be used creatively for waterfalls, light trails, or panning shots. Camera shake from unsteady hands also causes blur, which is why the 1/focal-length minimum shutter rule exists.
When should I use a fast versus a slow shutter speed?
Use a fast shutter speed (1/500 s and above) to freeze fast action such as sport, birds in flight, or splashing water. Use a moderate shutter speed (1/60 s to 1/250 s) for portraits and general handheld shooting where subject movement is limited. Use a slow shutter speed (1/30 s and below) when you want to show motion: smooth waterfalls, light trails from traffic, or creative panning blur around a moving subject. For shutter speeds below around 1/30 s, a tripod is strongly recommended to prevent camera shake blur.
Official sources
- Cambridge in Colour, Camera Exposure tutorial: Understanding Camera Exposure.
- Wikipedia, Shutter speed: Shutter speed (Wikipedia).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only.