What this Shutter Speed Calculator does
This Shutter Speed Calculator estimates the shutter duration that preserves a reference exposure after you change aperture or ISO. It uses the exposure triangle relationship among shutter time, lens aperture, and sensor sensitivity. Enter a shutter speed and settings that produced a reference exposure, then enter the new aperture and ISO. The result is an equivalent shutter speed in seconds under the same scene brightness.
The calculation can help when planning a photograph and comparing exposure settings. For example, you might want more depth of field by selecting a smaller aperture, or you may raise ISO to use a shorter exposure. The calculator gives a starting value for that adjustment. It does not inspect an image, measure available light, select a camera mode, or account for every camera and lens behavior.
Its formula is: reference shutter speed × (new aperture ÷ reference aperture)² × (reference ISO ÷ new ISO). Aperture values are f-numbers, while shutter speed is entered in seconds. Use positive, realistic settings. The result assumes the lighting stays constant and that the reference settings produced the exposure you want.
How to use the Shutter Speed Calculator
Start with settings from a photograph or a meter reading that represents the exposure you want to preserve. Enter the shutter duration in seconds. A shutter speed of 1/100 second is entered as 0.01 seconds. Then enter the reference aperture and ISO, followed by the aperture and ISO you plan to use.
- Reference shutter speed: The original exposure time, expressed in seconds.
- Reference aperture: The original lens f-number, such as f/4.
- New aperture: The f-number you intend to use for the next frame.
- Reference ISO: The original camera sensitivity setting.
- New ISO: The sensitivity setting for the adjusted exposure.
Read the result as a shutter duration in seconds. A smaller number means the shutter is open for less time. You can translate decimal seconds into a familiar fraction: 0.005 seconds is 1/200 second. Camera displays may round to common shutter increments, so the displayed setting can differ slightly from the calculated value.
For a worked example, suppose the reference exposure is 1/100 second at f/4 and ISO 100. You want f/5.6 and keep ISO 100. The calculation is 0.01 × (5.6 ÷ 4)² × (100 ÷ 100), which is 0.0196 seconds. That is approximately 1/51 second. A camera using standard increments might offer 1/50 second, a close equivalent.
How the Shutter Speed Calculator formula works
The aperture term is squared because the f-number describes a ratio, and light passing through the lens changes with the square of that ratio. Moving from f/4 to f/5.6 reduces the light reaching the sensor by about half, so the shutter must remain open about twice as long if ISO and scene brightness stay fixed. The formula represents that change with (new aperture ÷ reference aperture)².
The ISO term adjusts for sensitivity. If you double ISO while leaving the aperture unchanged, the same rendered brightness can generally be achieved with half the exposure time. The ratio reference ISO ÷ new ISO captures this inverse relationship. Combined with the aperture adjustment, it produces a shutter duration that aims to keep exposure equivalent.
For example, changing from ISO 100 to ISO 200 makes the ISO ratio 100 ÷ 200, or 0.5. If aperture is unchanged, the estimated shutter time is half the reference duration. If both aperture and ISO change, their factors multiply. This makes the result traceable to the settings entered rather than an unexplained recommendation.
The model assumes a steady scene and consistent metering. It does not include exposure compensation, neutral density filters, flash duration, lens transmission differences, or automatic processing. Use the result as a mathematical estimate, then check the camera meter, histogram, and image preview.
Use cases for the Shutter Speed Calculator
Photographers can use the estimate when changing one or more exposure settings while trying to preserve brightness. It is especially useful for planning a deliberate adjustment before taking a sequence of frames.
- Changing depth of field: Choose a narrower aperture for more of a scene to appear in focus, then estimate the longer shutter time needed.
- Managing movement: Raise ISO to help retain a faster shutter when photographing moving subjects in dimmer conditions.
- Tripod photography: Compare a preferred aperture with a practical shutter duration for a still subject.
- Learning exposure: Change one input at a time and observe how the estimated duration follows the exposure relationship.
- Planning image series: Keep a reference exposure and calculate corresponding settings for variations in aperture or ISO.
These uses support planning, but the right creative choice depends on the scene. A moving subject may need a short exposure to limit motion blur, while a stationary subject may tolerate a longer one. Camera stability, subject movement, and the intended visual effect remain part of the decision.
Other factors to consider when calculating shutter speed
A mathematically equivalent exposure does not guarantee an identical photograph. Shutter speed controls how movement appears. A longer duration can record blur from a moving subject or from camera shake. Stabilization can help with camera movement, but it cannot freeze a subject that is moving independently.
ISO also has image-quality consequences. Raising it can make a faster shutter possible, though the visible noise and dynamic range vary by camera and settings. Aperture affects depth of field and may affect lens sharpness or diffraction at smaller openings. Consider these tradeoffs alongside exposure.
Real cameras provide shutter settings in discrete steps, often full, half, or third stops. The calculator returns a continuous arithmetic estimate, so round to the nearest available setting and review the result. Light may also change between the reference and the new photograph. Re-meter if clouds move, a subject enters a different light, or the camera angle changes substantially.
Keep units consistent. Enter shutter time as seconds, not as the denominator of a fraction. Enter aperture as the f-number itself, such as 2.8 rather than a ratio. ISO values are numeric settings. These conventions help prevent a plausible-looking but incorrect answer.
Frequently asked questions
How do I enter a shutter speed shown as a fraction?
Convert the fraction to seconds before entering it. For 1/125 second, divide 1 by 125 to get 0.008 seconds. A speed of 1/30 second is about 0.0333 seconds. The result is also expressed in seconds and can be converted back to a fraction for camera use.
Does a higher f-number require a longer shutter time?
Yes, assuming scene brightness and ISO remain the same. A higher f-number means a smaller aperture opening, so less light reaches the sensor during each moment. The formula increases shutter duration by the square of the ratio between the new and reference f-numbers.
Can this estimate freeze motion?
No. It estimates equivalent exposure, not whether a moving subject will look sharp. Motion blur depends on subject speed, direction, distance, focal length, and the visual result you want. Choose a shutter duration for the motion, then adjust aperture or ISO and meter the scene.
Why does my camera show a slightly different shutter setting?
The calculation can produce durations between the camera's available increments. Cameras commonly round to a supported fraction such as 1/50 or 1/60 second. Metering differences, lens transmission, and exposure rounding can also affect the displayed value and final image.
Will the result remain accurate if the light changes?
The estimate assumes the scene brightness stays constant. If the lighting changes, the same settings may no longer preserve the same exposure. Take a fresh meter reading or review a test frame when conditions shift.