🔅Converters

Lux to EV Converter

Convert illuminance measured in lux to the exposure value (EV) at ISO 100. Useful for photographers to set aperture and shutter from a light meter.

Lux

EV @ ISO100

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From lux to exposure value

Lux measures illuminance — how much light reaches a surface — and is what a light meter or a phone sensor reports. Exposure value is the logarithmic scale photographers use to describe the same light. Converting between them lets you use a light meter app to choose aperture and shutter speed when you have no photographic meter.

Enter the reading in lux and the page returns the exposure value for ISO 100. The formula uses the incident metering calibration constant: the value is the base-two logarithm of the illuminance divided by 2.5. Each step of one on the scale corresponds to doubling or halving the light, which is exactly one stop of aperture or shutter.

A few references help calibrate intuition: a sunny midday sits around 15, an overcast day around 12, a well-lit interior around 7, and a room with a table lamp near 4. The sunny sixteen rule — f/16 with shutter speed equal to the inverse of the ISO — corresponds precisely to value 15, which you can check against the arithmetic.

Frequently asked questions

Does the value change with ISO?

The scale is anchored at ISO 100. For another ISO, add the base-two logarithm of the ratio: ISO 200 adds 1, ISO 400 adds 2. That is why a higher ISO allows a shorter shutter speed — the effective value rises.

Is measuring incident light the same as reflected?

No, and the calibration constant differs. A light meter measures light arriving at a point, with the sensor aimed at the source. A camera meter measures light reflected by the scene, which is why it errs on very bright or very dark subjects, which reflect different proportions.

Can I use my phone sensor?

You can, with a caveat: a phone's ambient light sensor is calibrated for adjusting screen brightness, not for photometry. It gets the order of magnitude right and errs enough to be unsuitable for critical work. As a quick reference in a difficult scene, it helps.

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