Exposure Value (EV) Calculator

Background.

Exposure value is the single number that collapses aperture and shutter speed into one measure of how much light a photograph is being given. This calculator returns it from the three settings you actually set — f-number, shutter speed and ISO — and will also run backwards, solving for the shutter speed or the f-number a stated light level demands.

It implements the APEX system exactly as the industry standard defines it, with nothing invented in between. CIPA DC-008 — the Exif specification every camera on the market writes its files to — sets out the whole system in five lines: the aperture value Av is 2·log₂(f-number), the shutter speed value Tv is −log₂(exposure time), the sensitivity value Sv is log₂(ISO ÷ 3.125), and the exposure value is simply Ev = Av + Tv = Bv + Sv. Every number this page returns is one of those four quantities or a difference between them, which is why it reports Av, Tv and Sv alongside the EV rather than hiding them.

The reason the system is additive is the reason photographers think in stops at all. Because each quantity is a base-2 logarithm, one unit of Av, Tv or Sv is exactly one stop — one doubling or halving of light. f/2 to f/2.8 is one unit of Av. 1/125 s to 1/250 s is one unit of Tv. ISO 400 to ISO 800 is one unit of Sv. Adding and subtracting these small integers is arithmetic you can do in your head on a windy hillside, which is precisely what the system was designed for in the 1960s and why it survives inside every Exif header written today.

The page reports two exposure values, and the distinction matters. The plain EV describes the settings: f/8 at 1/250 s is EV 13.97 whatever ISO you have set, because ISO is not part of Av + Tv. The EV100 figure describes the light: it takes the same exposure and asks what EV would have produced it at ISO 100, which is the convention every published light-level table uses. Shooting f/8 and 1/250 s at ISO 400 gives EV 13.97 in settings terms but only EV 11.97 of actual light — bright overcast, not sunshine. Confusing the two is why people find published EV charts disagree with their cameras.

Run in reverse, the page does the job a light meter does. Give it a light level in EV at ISO 100 along with an ISO and one of the two exposure settings, and it solves for the other: t = 2^(Av − Ev) for shutter speed, N = 2^(Av/2) for aperture. The answers will not land on a marked stop, because marked stops are themselves rounded — f/11 is really f/11.314, and 1/250 s is really 1/256 s in the ideal scale — so the page hands you the exact figure and tells you which way to round.

One thing is deliberately missing, and the reason is worth stating plainly. This page does not convert EV into lux or candelas per square metre. The standard's own brightness definition is Bv = log₂(B ÷ NK) with the note "N, K: constant" — the calibration constants are named but not valued, because ISO 2720 lets each meter manufacturer choose K within a range, and different makers do. Guessing one would put a systematic bias of up to a sixth of a stop into every light figure on the page. Instead the calculator stays inside what the standard defines exactly, and says so.

Below the widget you will find the full APEX derivation, three worked examples computed by hand, the standard's own aperture and shutter tables, why the sunny-16 rule is EV 14.64 rather than the EV 15 everyone quotes, and the metering ranges real cameras publish.

What is exposure value (ev) calculator?

Exposure value (EV) is a base-2 logarithmic measure of photographic exposure: one EV is one stop, a doubling or halving of the light reaching the sensor. It belongs to the APEX system — the Additive System of Photographic Exposure — which CIPA DC-008, the Exif standard, defines in Annex C as follows: ApertureValue Av = 2·log₂(f-number); ShutterSpeedValue Tv = −log₂(exposure time in seconds); BrightnessValue Bv = log₂(B ÷ NK); film sensitivity Sv = log₂(ASA ÷ 3.125); and Ev = Av + Tv = Bv + Sv. The left-hand equality is what the camera is doing, the right-hand equality is what the scene is doing, and a correct exposure is the statement that the two are equal. Because every term is a logarithm, the whole system is additive: open up one stop and add 1 to Av, halve the shutter speed and add 1 to Tv, double the ISO and add 1 to Sv. Note that EV as defined by Av + Tv contains no ISO term at all — f/8 at 1/250 s is the same EV on ISO 100 and ISO 6400. That is deliberate, because EV describes a camera setting, not a light level. To describe light you need a reference sensitivity, and the universal convention is ISO 100, giving EV100 = EV − log₂(ISO ÷ 100). Since Sv at ISO 100 is log₂(32) = 5 exactly, EV100 is simply Bv + 5. What EV cannot tell you on its own is the absolute light level in lux or cd/m², because that conversion requires the meter calibration constant K, which the standard names but does not fix and which varies between manufacturers by around a sixth of a stop.

How to use this calculator.

1. 01Pick the direction. "The EV of these settings" is the measuring direction — you have an exposure and want to know its value. The other two are the metering directions — you know the light and want the missing setting.
2. 02Enter the aperture as a plain f-number: 8 for f/8, 1.4 for f/1.4. It must be the real f-number of the lens, never a format-equivalent one, because equivalent apertures describe depth of field and not exposure.
3. 03Enter the shutter speed together with its unit. Type 250 with "fraction of a second" for 1/250 s, or 30 with "whole seconds" for a half-minute exposure. The unit selector is purely a typing convenience; it changes nothing in the maths.
4. 04Enter the ISO speed actually set on the camera. It never affects the plain EV, but it sets Sv and it is what converts the settings EV into the ISO 100 light level.
5. 05When solving for a setting, enter the scene light level as EV at ISO 100 — around 15 in bright sun, 12 under heavy overcast, 7 in a well-lit room, and negative outdoors at night. This is the figure published EV tables quote.
6. 06Read the EV and the EV100 as a pair. The first is what your settings are doing; the second is what the light is doing. If they differ, the difference is exactly log₂(ISO ÷ 100) — two stops at ISO 400, six at ISO 6400.
7. 07Round solved settings towards more exposure, not less, unless you are protecting highlights. A solved f/16.19 means f/16 rather than f/22; a solved 1/541.6 s means 1/500 s rather than 1/640 s. Digital sensors recover shadows far better than blown highlights, so the direction of the rounding is a real decision.
8. 08Use Av, Tv and Sv to plan changes in your head. Every unit is one stop, so trading two stops of shutter for two of aperture leaves the EV untouched — the whole point of the additive system, and the arithmetic behind every equivalent exposure.

The formula.

The whole system is four definitions and one identity, and CIPA DC-008 Annex C states them in exactly this form:

a) Av = 2·log₂(f-number) b) Tv = −log₂(exposure time) c) Bv = log₂(B ÷ NK) with B in cd/cm², N and K constants d) Sv = log₂(ASA ÷ 3.125) e) Ev = Av + Tv = Bv + Sv

Why 2·log₂ for the aperture? Because light gathered scales with the area of the entrance pupil, and the pupil diameter is f ÷ N, so illuminance goes as 1/N². Taking the base-2 log of 1/N² and flipping the sign gives 2·log₂N, which makes one unit of Av one stop. That is why the familiar aperture series runs in steps of √2: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 — Av 0 through 8, exactly as tabulated in the standard.

Shutter speed is simpler because exposure is directly proportional to time, so a single log suffices, negated so that shorter exposures score higher. The standard's own table maps Tv −5 to 30 s, Tv 0 to 1 s, Tv 3 to 1/8 s, Tv 8 to 1/250 s and Tv 11 to 1/2000 s. Note the marked speeds are rounded: Tv 8 is really 1/256 s, not 1/250 s, and Tv 7 is 1/128 s rather than 1/125 s. This calculator uses the number you actually type, so an entry of 1/250 s returns Tv 7.9658 rather than 8. That is not an error; it is the difference between the engraved scale and the ideal one.

Sensitivity uses the divisor 3.125 so that ISO 100 lands on Sv 5 exactly: 100 ÷ 3.125 = 32, and log₂32 = 5. ISO 400 gives Sv 7, ISO 51200 gives Sv 14.

The identity Ev = Av + Tv = Bv + Sv is the statement of correct exposure. The left side is what the camera does; the right side is what the scene and the film or sensor do. From it, the ISO-100 reference falls out with no further assumptions: for a fixed scene brightness Bv, two ISO speeds differ only in Sv, so

EV₁₀₀ = Ev − log₂(ISO ÷ 100)

Solving backwards is then algebra. Given a light level and an aperture,

Tv = Ev − Av t = 2^(Av − Ev)

and given a light level and a shutter speed,

Av = Ev − Tv N = 2^(Av ÷ 2)

One thing the identity cannot give you is an absolute light level. Line (c) contains the constants N and K, which the standard names and does not value. ISO 2720:1974, the standard governing exposure meters, permits each manufacturer to pick K within a range, and the values in common use differ by about a sixth of a stop. A calculator that prints a lux figure has silently chosen one of them for you. This page does not, and instead confines itself to the quantities the standard pins down exactly.

Finally, a note on sunny 16, which is usually quoted as EV 15. The rule states that in bright sun you shoot f/16 at a shutter speed of 1/ISO. At ISO 100 that is f/16 and 1/100 s, giving Av 8 and Tv 6.644, so Ev = 14.64 — not 15. The round number comes from rounding 1/100 s up to the tabulated 1/128 s. This page returns 14.64 because that is what the settings compute to, and reports the rounding rather than absorbing it.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Camera & Imaging Products Association / JEITA. CIPA DC-008-2012 (JEITA CP-3451C), 'Exchangeable image file format for digital still cameras: Exif Version 2.3', Annex C — APEX Units. The source for every equation on this page: 'a) ApertureValue (Av) = 2 log 2 (F number); b) ShutterSpeedValue (Tv) = - log 2 (exposure time); c) BrightnessValue (Bv) = log2 ( B/NK ) Note that: B:cd/cm2, N,K: constant; d) Film sensitivity (Sv) = log2 ( ASA / 3.125 ); e) Ev = Av + Tv = Bv + Sv.' Annex C also carries Table 52, 'Typical APEX Values', mapping Av 0–10 to f/1–f/32 and Tv −5–11 to 30 s–1/2000 s.
2. [2]Nikon Corporation. D7500 Online Manual, Technical Notes — Specifications. Gives the exposure metering range as '0 – 20 EV' for matrix, centre-weighted and highlight-weighted metering and '2 – 20 EV' for spot metering, qualified 'at ISO 100, f/1.4 lens, 20 °C/68 °F'; the shutter range as '1/8000–30 s in steps of 1/3 or 1/2 EV, bulb, time, X250'; and sensitivity as 'ISO 100–51200 in steps of 1/3 or 1/2 EV'.
3. [3]Canon Inc. EOS R7 Product Manual, Specifications. Gives the metering range as 'EV –2 to 20' for still photo shooting and 'EV 0 to 20' for movie recording; shutter speeds of '1/8000–30 sec., Bulb' with the mechanical shutter and '1/16000, 1/8000–30 sec., Bulb' with the electronic shutter; and 'ISO 100–32000' with an expanded 'H (equivalent to ISO 51200)'.
4. [4]Canon Inc. EOS R6 Product Manual, Specifications — Image Sensor. Gives the full-frame sensor size as 'Approx. 35.9×23.9 mm'. Cited here in support of the point that exposure value is a property of the settings and the light and not of the sensor format: the same f-number and shutter speed give the same EV on any sensor size.
5. [5]International Organization for Standardization. ISO 2720:1974, 'Photography — General purpose photographic exposure meters (photoelectric type) — Guide to product specification'. The standard governing the calibration constant K that CIPA DC-008 Annex C refers to but does not value. It permits manufacturers to select K within a range, which is why this calculator publishes no luminance or illuminance figure. Cited without a link: the ISO catalogue entry returned HTTP 403 when fetched, and the standard itself is paywalled.
6. [6]Nasse, H. H. (2010). 'Depth of Field and Bokeh.' Carl Zeiss Camera Lens Division. Source for the warning that a sensor-format-converted f-number 'would be incorrect as an exposure parameter' — the reason this page insists on real f-numbers rather than 35 mm-equivalent ones in the aperture field.