Exposure Calculator
The exposure value of an aperture, shutter speed and ISO, and the equivalent exposure when you change one setting: get the shutter, aperture or ISO that keeps the same brightness.
Result…
How it's calculated
EV = log₂(N² ÷ t) (N = f-number, t = shutter time in seconds) EV at ISO 100 = log₂(N² ÷ t) − log₂(ISO ÷ 100) Same exposure: N₂² ÷ t₂ = (N₁² ÷ t₁) × (ISO₂ ÷ ISO₁) New shutter t₂ = t₁ × (N₂ ÷ N₁)² × (ISO₁ ÷ ISO₂) New aperture N₂ = N₁ × √((t₂ ÷ t₁) × (ISO₂ ÷ ISO₁)) New ISO = ISO₁ × (N₂ ÷ N₁)² × (t₁ ÷ t₂)
Works out the exposure value (EV) of a camera setting: EV = log₂(N² ÷ t), where N is the f-number and t the shutter time in seconds, and the EV at ISO 100 by taking off log₂(ISO ÷ 100). One EV is one stop, a doubling or halving of the light. Change one setting and it finds the shutter speed, aperture or ISO that keeps the same exposure, rounded if you like to the nearest third, half or full stop, and shows how far the rounding moves the exposure. Full-stop results are also shown as the numbers marked on lenses and shutter dials. The scene luminance uses the meter constant K = 12.5 that the exposure value article gives.
Your numbers, step by step
About the Exposure calculator
This exposure calculator gives the exposure value (EV) of any aperture, shutter speed and ISO, and finds equivalent exposures when you change one setting. Enter the f-number, the shutter speed as a fraction such as 1/125 or in seconds, and the ISO. It returns EV = log₂(N² ÷ t), the EV at ISO 100 that describes the light in the scene, and the aperture, time and speed values behind it. Switch to “Shutter speed”, “Aperture” or “ISO” and it solves for the missing setting that keeps the same brightness after you open up the lens, stop down or raise the ISO. Results can be rounded to the nearest third, half or full stop, the rounding error is shown in stops, and full-stop answers also appear as the numbers marked on lenses and shutter dials. Photographers use it for manual exposure, ND filters and long exposures.
How to use it
- Choose what to work out: “Exposure value”, “Shutter speed”, “Aperture” or “ISO”.
- Enter your metered setting: the “Aperture (f-number)”, the “Shutter speed” (such as 1/250 or 2) and the “ISO”.
- For an equivalent exposure, enter the settings you want to change, such as the “New aperture (f-number)” and “New ISO”.
- Pick “Round to” 1/3, 1/2 or full stops to match your camera, or “Exact”.
- Read the new setting, the rounding difference in stops and the EV at ISO 100 in the steps.
Worked example
The meter reads f/8 at 1/125 s and ISO 100. EV = log₂(8² ÷ 1/125) = log₂(8000) = 12.97, so the EV at ISO 100 is about 13. To blur the background you open up to f/2.8 at the same ISO. The new shutter time is 1/125 × (2.8 ÷ 8)² = 0.00098 s. Rounded to the nearest full stop that is 2⁻¹⁰ s, marked 1/1000 s on the dial, only 0.005 stop darker than exact.
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Questions people ask
How do I calculate exposure value?
EV = log₂(N² ÷ t), where N is the f-number and t the shutter time in seconds. To get the EV at ISO 100, take off log₂(ISO ÷ 100): for example 2 at ISO 400.
How do I find an equivalent exposure?
Keep N² ÷ t in step with the ISO: each stop you open the aperture halves the shutter time, and each doubling of ISO also halves it. The calculator solves for the shutter, aperture or ISO you leave out.
What is a stop in photography?
One stop doubles or halves the light. On the aperture it is a factor of √2 in the f-number (f/5.6 to f/8), on the shutter a factor of 2 in time, and on ISO a factor of 2 in speed.
Why does the calculator say 1/128 s instead of 1/125 s?
Exact third and full stops from 1 s are powers of two, so 2⁻⁷ s is 1/128 s. Cameras mark rounded numbers such as 1/125 s; the gap is far too small to see.
Does the same EV always give the same picture?
No. Equal exposures have the same brightness, but the aperture changes the depth of field and the shutter speed the motion blur. The depth of field calculator shows the sharp zone for an aperture.
What are the limits of this exposure calculation?
It assumes the meter reading is right for the scene and ignores reciprocity failure, which makes very long film exposures need extra time. Filters, lens transmission (T-stops) and bellows extension change the light reaching the sensor too.