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Exposure Value Calculator

Computes the exposure value EV = log2(N²/t) of a camera setting, the ISO-100 normalised light value EV100, an illuminance estimate in lux (incident-meter calibration C = 250) and a description of the lighting the settings correspond to.

When to use

You want to express aperture, shutter and ISO as a single exposure number, estimate how bright a scene is, or check settings against the Sunny 16 rule.

Do not use when: You want to translate one setting into another with the same exposure (use equivalent-exposure) or to work out a long exposure with an ND filter (use nd-filter-exposure).

Formula

ev = log2(aperture_f² / shutter_seconds); ev100 = ev − log2(iso / 100); illuminance_lux = 250 × 2^ev / iso = 2.5 × 2^ev100

EV definition per the APEX system; the lux relation follows the ISO 2720 incident-meter equation EV = log2(E·S / C) with C = 250 lx·s, so it assumes a correctly exposed scene of average reflectance and is only an estimate (±1 EV).

Inputs

ParameterTypeUnitRequiredDescription
aperture_fnumberyesLens f-number, e.g. 2.8 for f/2.8. Range: ≥ 0.5, ≤ 128
shutter_secondsnumbersyesExposure time in seconds. Enter fractions as decimals: 1/125 s = 0.008, 1/60 s = 0.016667, 1/1000 s = 0.001. Range: > 0, ≤ 86400
isonumberdefault 100ISO sensitivity setting (100, 200, 400, ...). Range: ≥ 1, ≤ 10000000

Outputs

OutputTypeUnitDescription
evnumberEVlog2(aperture_f² / shutter_seconds), independent of ISO.
ev100numberEVev − log2(iso / 100): the scene brightness these settings expose correctly.
illuminance_lux_estimatenumberlx2.5 × 2^ev100 lux, the incident-light meter relation with calibration constant C = 250 (average subject reflectance assumed).
lighting_descriptionstringScene type that corresponds to ev100 (EV 15 bright sun, EV 12–13 overcast, EV 7–8 bright indoor, EV 4–6 dim indoor, EV ≤ 0 night).
shutter_textstringThe shutter speed as a conventional fraction, e.g. '1/125 s'.

Example

Sunny 16: f/16, 1/125 s, ISO 100: {"aperture_f":16,"shutter_seconds":0.008,"iso":100} → {"ev":15,"ev100":15,"illuminance_lux_estimate":80000,"lighting_description":"Bright or hazy sun with distinct shadows (EV 15, Sunny 16 rule)"}

Indoor: f/2.8, 1/60 s, ISO 400: {"aperture_f":2.8,"shutter_seconds":0.016667,"iso":400} → {"ev":8.9,"ev100":6.9,"lighting_description":"Bright indoor lighting: offices, shops, well-lit rooms (EV 7–8)"}

GET https://tttkmbb.com/api/v1/calculate/exposure-value?aperture_f=16&shutter_seconds=0.008&iso=100

Machine access

Sources

FAQ

What is the difference between EV and EV100?

EV only combines aperture and shutter. EV100 also removes the ISO, so it describes the light in the scene: f/2.8 at 1/60 s is EV 8.9 at any ISO, but at ISO 400 it exposes an EV100 6.9 scene correctly.

Why is the lux value only an estimate?

Camera meters read reflected light and are calibrated for mid-grey subjects; the conversion assumes that and uses C = 250, while meter makers use constants between 240 and 340, so real illuminance can differ by about ±1 EV.

How do I enter 1/250 s?

As a decimal: 1/250 = 0.004 s. Common values: 1/1000 = 0.001, 1/500 = 0.002, 1/125 = 0.008, 1/60 = 0.016667, 1/30 = 0.033333.

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