An exposure value calculator collapses three separate camera settings into one number. Aperture controls how wide the light path is, shutter speed controls how long it stays open, and ISO controls how much the resulting signal is amplified. Any of the three can be traded against the others without changing how bright the final image is, and exposure value — EV — is the single figure that describes where that trade currently sits.
Arb Digital publishes this in its free tools library beside the depth of field calculator, which handles what aperture does to sharpness rather than to brightness, and the hyperfocal distance calculator for focus planning. This page performs exposure arithmetic on values you type. It does not meter a scene and it does not read a photograph — recovering the settings from a file you already shot is the job of the image metadata viewer.
What This Exposure Value Calculator Does
It converts an aperture and shutter speed into a raw exposure value, then adjusts for ISO to give the EV100 figure — the exposure value referenced to ISO 100, which is the convention almost all published EV tables and light meters use. It also returns the shutter speed that would keep the exposure identical at a different aperture, and the corrected shutter time after adding a neutral density filter of a given strength.
The implied scene luminance is included because EV is only half a measurement without it. An exposure value describes a camera setting, not a scene. It becomes a statement about the world only when you assume the setting produces a correct exposure, at which point the luminance of the metered subject can be recovered. That conversion uses a reflected-light calibration constant, and the value used here is K = 12.5, the figure adopted by most major camera manufacturers. Meters using K = 14 will disagree by about a sixth of a stop.
The three bars break the EV100 figure into its components in stops, which makes the arithmetic visible. Once you can see that f/8 contributes six stops and 1/250 contributes just under eight, the reciprocity between the settings stops being abstract.
How to Use It
- Enter the aperture and shutter speed you are actually using. The shutter units selector handles both fractions of a second and long exposures, so 250 in fraction mode means one two-hundred-and-fiftieth while 30 in seconds mode means a half-minute exposure.
- Set the ISO to whatever the camera is on. The page reports both the raw EV for that ISO and the EV100 reference, and the difference between them is exactly the number of stops your ISO sits above or below 100.
- Use the target aperture field to find an equivalent exposure. Enter the aperture you would rather shoot and read the shutter speed that keeps brightness unchanged. This is how you trade depth of field against motion blur without re-metering.
- Select a filter to get the corrected time. Neutral density filters are specified in stops, and each stop doubles the exposure time. Six stops multiplies it by sixty-four.
- Sanity-check against the luminance figure. If the implied luminance for a bright outdoor scene comes out at a few dozen candela per square metre, an input is wrong somewhere.
The Formula / How It's Calculated
Exposure value is defined as a base-two logarithm of the ratio of aperture squared to exposure time:
EV = log₂(N² ÷ t), where N is the f-number and t is the exposure time in seconds. Because the logarithm is base two, every whole EV step is exactly one stop — a doubling or halving of the light reaching the sensor.
That raw figure depends on ISO, so the conventional reference is ISO 100: EV100 = log₂(N² ÷ t) − log₂(ISO ÷ 100). When the ISO is 100, the two figures are identical, which is why published EV tables never mention ISO explicitly.
Scene luminance follows from the reflected-light meter equation L = K × N² ÷ (t × ISO) with K = 12.5, and incident illuminance from E = C × N² ÷ (t × ISO) with C = 250. Equivalent exposures come from holding the raw EV constant: t₂ = N₂² ÷ 2^EV. Filter correction is a straight doubling per stop: t_filtered = t × 2^stops.
Worked example, matching the values the page loads with. At f/8, 1/250 second and ISO 200: N² ÷ t = 64 × 250 = 16,000, so the raw EV is log₂(16,000) = 13.97. Subtracting log₂(200 ÷ 100) = 1 stop gives EV100 = 12.97. The implied luminance is 12.5 × 16,000 ÷ 200 = 1,000 cd/m² and the implied illuminance is 250 × 16,000 ÷ 200 = 20,000 lux. Switching to f/2.8 at the same exposure needs t = 7.84 ÷ 16,000 = 1/2,041 second. Adding a six-stop filter to the original settings extends 1/250 by a factor of 64, giving 0.256 seconds. Stanford's CS178 notes on the variables that affect exposure work through the same three controls interactively.
Why EV100 Is the Number Worth Quoting
The raw exposure value has an awkward property: it describes the camera setting rather than the light. Two photographers standing side by side in the same light, one at ISO 100 and one at ISO 3200, will be using settings five stops apart and will therefore compute raw EVs five apart. Both are correct, and neither number tells you anything useful about the scene on its own.
Referencing everything to ISO 100 fixes this. EV100 is a property of the light, provided the exposure is correct, and it is directly comparable between cameras, between photographers and against published tables. A moonlit landscape is around EV100 −3. An average indoor room is around EV100 6 to 7. A bright, sunlit scene sits near EV100 15, which is where the sunny sixteen rule comes from: at f/16 and one over the ISO for shutter speed, the arithmetic lands you almost exactly there.
Handheld light meters follow this convention, which is why their EV readouts require an ISO to be dialled in first. Cameras that display EV in an exposure compensation context are usually showing something different again — deviation from the metered value in stops — and confusing those two uses of the same letters is a frequent source of misread advice.
The practical rule is simple. When comparing scenes, quote EV100. When describing a specific camera setting, quote the aperture, shutter and ISO explicitly. A bare "EV 14" with no ISO attached is ambiguous and should be treated as such.
Where Reciprocity Breaks Down
The premise underlying every equivalent exposure is that the aperture, shutter and ISO trade cleanly against each other. Halve the light through the aperture, double the time, and the exposure is identical. This is called reciprocity, and it is close enough to true across the normal working range that photographers rely on it without thinking. It has real limits.
Film exhibited reciprocity failure at long exposures. Past roughly a second, silver halide crystals stopped responding linearly and required substantially more light than the arithmetic predicted, with the correction rising steeply — a calculated thirty second exposure might need two minutes. Colour films failed at different rates in different layers, which shifted the colour balance as well as the density. Digital sensors do not have this problem in the same form, which is why a modern long exposure calculation holds where a film one would not.
Digital sensors have their own departures. Very long exposures accumulate dark current, adding heat-dependent noise that no exposure adjustment corrects. Very short exposures with mechanical shutters can suffer from uneven travel across the frame. Electronic shutters introduce rolling shutter distortion on moving subjects, which is not an exposure error but is certainly a trade-off the arithmetic does not show.
Strong neutral density filters bring a third problem. Most so-called neutral filters are not perfectly neutral, and ten and fifteen stop filters commonly introduce a visible colour cast, usually magenta or cyan. Their marked density is also frequently approximate — a filter sold as ten stops may measure nine and a half or eleven — so a calculated time is a starting point that should be verified with a test frame rather than trusted outright.
Aperture itself is not exactly what the barrel says. The engraved f-number is a geometric ratio, but the light actually transmitted is reduced by absorption and reflection inside the lens. Cinema lenses are marked in T-stops, which are measured transmission values, precisely because a director of photography matching two lenses cannot afford a third of a stop of discrepancy between them. A complex zoom can lose noticeably more light than a simple prime at the same marked aperture.
Reading the Exposure Triangle Through the Bars
The three bars on this page are not decoration; they are the equation rearranged into something you can see. The aperture term is log₂(N²), which is simply twice log₂(N), and that is the reason the f-number sequence looks so strange. Each full stop multiplies the f-number by the square root of two: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16. The numbers seem arbitrary until you notice they are powers of √2 rounded, and that squaring them gives a clean doubling.
The shutter term is log₂(1 ÷ t), which behaves much more intuitively because the standard shutter sequence really is a straight doubling: 1/60, 1/125, 1/250, 1/500. The slight irregularities — 1/125 rather than 1/128 — are historical rounding, not a different mathematics.
The ISO term is subtracted, and this is where the sign confuses people. Raising ISO does not add light. It amplifies whatever signal was collected, which means the same brightness is achieved from less light, which means the EV100 corresponding to your settings goes down. Higher ISO, lower EV100, dimmer scene. The bar shows the offset in stops so the direction is unambiguous.
The international standard governing how ISO speed is assigned and reported for digital cameras is ISO 12232:2019, which defines exposure index, ISO speed ratings, standard output sensitivity and recommended exposure index. Manufacturers do not all choose the same one of those definitions, which is why two cameras set to the same nominal ISO can differ by a third of a stop in practice.
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See Web Design Services Talk to Arb DigitalCommon Mistakes to Avoid
- Quoting an EV without stating the ISO — the raw figure describes a camera setting, and only the ISO 100 reference is comparable between cameras or against published tables.
- Confusing exposure value with exposure compensation — a camera's plus or minus figure in stops is deviation from the meter, not an absolute exposure value, despite both being written as EV.
- Entering the shutter speed in the wrong units — 30 in fraction mode is one thirtieth of a second, while 30 in seconds mode is nine hundred times longer.
- Trusting a strong filter's marked density exactly — ten and fifteen stop filters frequently deviate by half a stop or more and often add a colour cast, so treat the calculated time as a starting point.
- Assuming marked f-numbers equal transmitted light — the f-number is geometric, while T-stops are measured, and a complex zoom can transmit noticeably less than a prime at the same marking.
Related Free Tools From Arb Digital
Work out what an aperture does to sharpness with the depth of field calculator and the hyperfocal distance calculator, convert settings between bodies with the crop factor calculator, and plan framing with the camera field of view calculator. Time an interval sequence with the time lapse calculator, work through lens geometry with the thin lens equation calculator, and size storage with the image file size calculator. Everything else is in the free online tools hub.
Frequently Asked Questions
The headline number is EV100, meaning exposure value referenced to ISO 100, which is the convention used by handheld meters and published tables. The raw exposure value for whatever ISO you entered is shown separately, and the two differ by exactly the number of stops your ISO sits above or below 100.
No, although both are written in EV units. Exposure compensation is a relative adjustment expressed in stops away from what the meter chose. Exposure value as calculated here is an absolute figure derived from aperture, shutter and ISO, independent of any metering decision.
Because ISO amplifies the signal rather than adding light. A higher ISO reaches correct exposure from less light, so the same aperture and shutter combination corresponds to a dimmer scene, and the ISO 100 reference figure falls accordingly.
K = 12.5, the reflected-light constant adopted by most major camera manufacturers, giving luminance in candela per square metre. The illuminance figure uses the incident constant C = 250 for lux. Meters calibrated to K = 14 will read about a sixth of a stop differently.
Yes, because exposure depends on the f-number squared. Each full stop multiplies the f-number by the square root of two, which is why the sequence runs 1.4, 2, 2.8, 4, 5.6, 8 — squaring those gives a clean doubling of the aperture term.
Not in the film sense. Silver halide required substantially more light than the arithmetic predicted past about a second, and colour layers failed at different rates. Digital sensors respond linearly over a much wider range, though very long exposures do accumulate heat-dependent dark current noise that no exposure adjustment removes.
The arithmetic is exact — each stop doubles the time — but the filter may not be. Ten and fifteen stop filters commonly deviate by half a stop or more from their marked density and often add a colour cast, so shoot a test frame and adjust rather than trusting the calculated time outright.
Because the governing standard permits more than one method of assigning a speed rating, including standard output sensitivity and recommended exposure index, and manufacturers do not all pick the same one. Differences of around a third of a stop between bodies at the same nominal ISO are ordinary.
