How this instrument works
Exposure value (EV) is a single number, on a base-2 logarithmic scale, that combines aperture and shutter speed into one figure describing how much light reaches the sensor or film for a given exposure. It comes from the APEX (Additive System of Photographic Exposure) system formalized in the 1960 ASA standard, and it's why photographers can talk about 'EV 12' or 'EV 15' as shorthand for a whole family of equivalent aperture-and-shutter combinations.
Because EV is logarithmic in base 2, each whole EV step represents a doubling or halving of light -- exactly one 'stop' in photographic terms. f/8 at 1/125s and f/11 at 1/60s produce nearly the same EV, because closing the aperture by roughly one stop and slowing the shutter by roughly one stop cancel each other out; EV describes the total light captured, independent of which of the two settings produced it.
This calculator solves for EV given a specific aperture and shutter speed you've already chosen or measured -- useful for checking a metering reading, comparing two exposures, or matching an EV chart value like 'EV 15, bright sun.' If you instead know your target EV and want to solve for the shutter speed at a given aperture, this site's shutter speed calculator runs the same formula in reverse.
- Enter f-number (f/N) -- the aperture you're shooting at, e.g. 8 for f/8.
- Enter Shutter speed (seconds) -- as a decimal, e.g. 0.008333 for 1/120s (divide 1 by the shutter fraction's denominator).
- Read Exposure value (EV) beneath the inputs -- the combined brightness figure on the standard EV scale.
- Compare your result against a standard EV chart, widely published in photography references, to gauge whether a scene is bright daylight, overcast, or dim interior lighting.
Worked example -- f/8 at 1/120s
Enter an f-number of 8 and a shutter speed of 0.008333 seconds (1/120s). Exposure value comes out to log2(8^2 / 0.008333) = log2(7680.3) = 12.9069 -- close to EV 13 on a standard exposure chart, the value associated with bright, hazy sunlight.
Questions
What does an exposure value (EV) of 0 mean?
EV 0 is defined as f/1.0 at 1 second -- a specific, quite dim reference exposure combination. Positive EV values indicate brighter scenes, needing less exposure, while negative EV values describe very dim scenes needing long exposures even at a wide-open aperture, such as night photography under starlight.
Why do different aperture and shutter combinations give the same EV?
Because EV measures total light captured, not the specific settings used to capture it -- opening the aperture by one stop doubles the light let through, and halving the shutter speed independently doubles the light let through too, so the two changes can offset each other and leave EV unchanged. f/8 at 1/125s and f/5.6 at 1/250s land on very nearly the same EV for this reason, the basis of the 'exposure triangle' photographers juggle between aperture, shutter speed and ISO.
How do I convert a shutter fraction like 1/125 into decimal seconds?
Divide 1 by the fraction's denominator: 1/125s becomes 1 / 125 = 0.008 seconds, and 1/60s becomes 1 / 60 = 0.016667 seconds. Most camera shutter speeds are published as these fractions specifically because they're easy to read at a glance, but the underlying calculation always needs the decimal-seconds form.
How does ISO fit into exposure value if this formula doesn't use it?
The formula here computes the base EV from aperture and shutter speed only, calibrated to ISO 100 by convention -- it describes the scene's actual light level, independent of camera settings. Raising ISO lets a camera produce a correctly-exposed image at a different aperture/shutter combination than the base EV would otherwise require, but it does so by amplifying the sensor's signal, not by changing how much light physically reached it.
How do I go the other way and find shutter speed for a target EV?
Use this site's dedicated shutter speed calculator, which solves the identical EV formula algebraically for shutter speed instead of EV -- enter your aperture and a target exposure value, from a chart or a light meter reading, and it returns the exact shutter speed in seconds needed to hit that EV at your chosen aperture.