Shutter Speed Calculator

Find the shutter speed that keeps your exposure equivalent when you change aperture or ISO, using the standard exposure reciprocity formula, plus a handheld-shake safety check.

Quick Facts

Reciprocity formula
t2 = t1 x (N2/N1)^2 x (ISO1/ISO2)
Keeps total light on the sensor constant when aperture or ISO changes.
Handheld rule of thumb
Shutter speed >= 1 / focal length (mm)
Classic guideline to avoid camera shake without a tripod or stabilization.

Your Results

Calculated
New shutter speed
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Equivalent exposure at new settings
Stop change
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Net stops vs. base shutter speed
Exposure value
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Effective EV at ISO 100
Handheld check
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Vs. 1/focal length guideline

Ready

Enter your base and new settings, then press Calculate.

Understanding the shutter speed reciprocity formula

Shutter speed, aperture (f-number), and ISO form the "exposure triangle" — the three settings that together determine how bright a photo turns out. Change one and, to keep the same overall brightness, you must change the others by an offsetting amount. This calculator answers the most common version of that question: given a base exposure that already looks right, what shutter speed keeps the exposure equivalent after you pick a new aperture and/or a new ISO?

The exposure equivalence formula

Exposure (the total light reaching the sensor) is proportional to shutter time × ISO ÷ aperture². Setting the base exposure equal to the new exposure and solving for the new shutter time gives:

t2 = t1 × (N2 / N1)² × (ISO1 / ISO2)

where t1 and N1 are the base shutter speed and f-number, ISO1 is the base ISO, and N2 and ISO2 are the new aperture and ISO. The squared aperture ratio appears because the amount of light passing through a lens opening scales with the area of the aperture, which scales with the square of the f-number's reciprocal.

Reading the stop change

A "stop" is a doubling or halving of light — the standard unit photographers use to compare settings. The calculator reports the net stop change as log₂(t2 / t1): a positive number means the new shutter speed is slower (more time, needed because the new settings let in less light), and a negative number means it is faster. Full-stop aperture values (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22) each let in half the light of the previous one; full-stop ISO values (100, 200, 400, 800, 1600...) each double sensitivity.

Handheld shutter speed guideline (the reciprocal rule)

A long-standing rule of thumb says a shutter speed should be at least as fast as 1 ÷ focal length (in seconds) to avoid visible blur from hand tremor when shooting without a tripod or image stabilization — for example, 1/50s or faster with a 50mm lens, or 1/200s or faster with a 200mm telephoto. This calculator checks the new shutter speed against that threshold using the focal length you enter. Optical or in-body stabilization typically buys back two to five stops, letting you shoot slower than the raw guideline; a moving subject may still blur even at a "safe" handheld speed.

Typical shutter speeds and what they're used for

  • 1/2000s–1/4000s: freezing fast sports or wildlife action.
  • 1/500s–1/1000s: general action, kids, pets in motion.
  • 1/125s–1/250s: everyday handheld shooting, portraits.
  • 1/30s–1/60s: handheld with a steady grip or stabilization, panning shots.
  • 1/4s–2s: tripod-based low light, light waterfalls/motion blur.
  • 5s and longer: long-exposure night, astro, and light-trail photography.

Frequently Asked Questions

What is the shutter speed reciprocity formula?
To hold the exposure constant when aperture or ISO changes, the new shutter speed is t2 = t1 × (N2/N1)² × (ISO1/ISO2), where t1/N1/ISO1 are the base settings and N2/ISO2 are the new aperture and ISO. It comes from the exposure law: exposure is proportional to (shutter time × ISO) ÷ aperture².
Why does changing ISO affect the required shutter speed?
ISO sets the sensor's sensitivity. Doubling ISO doubles sensitivity, so the sensor needs half as much light for the same brightness — one stop faster shutter speed. Halving ISO means the shutter must stay open twice as long to compensate.
What does the "stop change" number mean?
A stop is a doubling or halving of light. +1 stop means twice the exposure time (or twice the light); −1 stop means half. The calculator reports the net stops between your base and new shutter speed after both the aperture and ISO changes are combined.
What is the 1/focal length rule for handheld shooting?
A common guideline keeps shutter speed at or faster than 1 ÷ focal length in millimeters, expressed in seconds — 1/50s or faster for a 50mm lens — to avoid visible camera shake handheld. Image stabilization or a tripod lets you go slower than this guideline.

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