Lux to EV Calculator: Scene Light to Camera Settings
Convert illuminance in lux or foot-candles to exposure value (EV at ISO 100 and at your ISO), or EV back to lux. Get the shutter speed at your aperture, the ISO needed for a hand-held shutter speed, a full equivalent-exposure table and an EV-to-lux chart, using the incident-meter constant C = 250.
How it works
EV₁₀₀ = log₂(E × 100 ÷ C), with E in lux and C = 250 for a flat receptor; E = C × 2^EV₁₀₀ ÷ 100; EV at ISO S = EV₁₀₀ + log₂(S ÷ 100); shutter t = N² ÷ 2^EV_S; ISO needed for time t = N² × C ÷ (E × t); 1 foot-candle = 10.764 lux
Exposure value (EV) is one number for how much light a camera has to handle. Each step of 1 EV is a doubling of light, and every aperture and shutter pair with the same EV gives the same exposure. An incident-light meter turns illuminance into EV with the equation E × S ÷ C = 2^EV, where E is lux, S is the ISO speed and C is the meter calibration constant. ISO 2720 allows 240–400 for a flat receptor and 320–540 for a dome; 250 (flat) is the figure most calculators and phone apps use, and Minolta and Sekonic domes sit around 320–340. With C = 250, 2.5 lux is EV 0 at ISO 100, 2,560 lux is EV 10 and full sun at about 100,000 lux is EV 15.3, which is where the sunny 16 rule comes from. The calculator adds the stops from your ISO, then solves t = N² ÷ 2^EV for the shutter time at your aperture and rounds it to the nearest marked 1/3-stop speed. It also turns the problem round and gives the ISO you need for a chosen shutter speed, which is the quickest way to judge a venue: 200 lux at f/2.8 and 1/125 s needs about ISO 1,250. The equivalent-exposure table lists every full-stop aperture from f/1.4 to f/22, and the EV ladder shows the lux for each EV with C applied. Limits: lux readings from phone apps can be off by a third of a stop or more, the result assumes an average subject in the measured light, and film reciprocity failure for exposures over about 1 s is not included.
Worked example
A bright office at 400 lux, shooting at ISO 800 and f/2.8 with C = 250: EV₁₀₀ = log₂(400 × 100 ÷ 250) = log₂(160) = 7.32, shown as EV 7.3. ISO 800 is 3 stops above ISO 100, so the EV at ISO 800 is 10.32. The shutter time is 2.8² ÷ 2^10.32 = 7.84 ÷ 1,280 = 0.006125 s, about 1/163 s, and the nearest marked speed is 1/160 s. To shoot at 1/125 s instead you need ISO 7.84 × 250 ÷ (400 × 1/125) = 612.5, so set ISO 640. 400 lux is 37.2 foot-candles, and an 18% grey card in that light has a luminance of 22.9 cd/m².
Sources: Wikipedia: Light meter, calibration constants (ISO 2720) · colour-hdri: illuminance_to_exposure_value (C = 250) · Analog.cafe: exposure values, stops and lux
Frequently asked questions
How many lux is EV 0, EV 10 and EV 15?
With the usual flat-receptor constant C = 250 at ISO 100: EV 0 is 2.5 lux, EV 10 is 2,560 lux and EV 15 is about 81,900 lux. Each extra EV doubles the lux.
Why does a dome meter read differently?
A dome (hemispherical) receptor also collects light from the sides and is calibrated with C ≈ 320–340, so for the same lux figure it suggests about a third to half a stop more exposure than C = 250. Enter the constant that matches your meter under More options.
Is EV the same as LV?
Light value (LV) is EV at ISO 100, the first result here. Camera menus often use “EV” for exposure compensation or for EV at the current ISO, which is the second result.
Can I use a phone lux meter app?
Yes for planning. Hold the phone at the subject with the sensor facing the camera. Phone light sensors are not calibrated like a photographic meter, so bracket by ±1/3 to ±2/3 stop when it matters.
How do I convert foot-candles to EV?
Choose foot-candles in the unit list, or multiply by 10.764 to get lux first. 100 fc is 1,076 lux, which is about EV 8.75 at ISO 100.
What ISO do I need for an indoor event?
Use the ISO-needed result. At 200 lux, f/2.8 and 1/125 s you need about ISO 1,250; at 100 lux in a living room it doubles to about ISO 2,500.
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This calculator is for educational purposes. Double-check important results before acting on them.