Exposure Value EV Calculator

Exposure value collapses two controls into one number: EV is the base-2 logarithm of the f-number squared divided by the shutter time in seconds. The logarithm makes the scale additive, so every whole step doubles or halves the light and any aperture and shutter pair on the same EV gives the same exposure. EV 0 is defined as f/1.0 for one second at ISO 100. Because that reference sensitivity is baked in, changing ISO shifts the value by log2 of the ISO divided by 100, so ISO 800 adds exactly three stops. The aperture term is squared because pupil area, not diameter, sets the light-gathering rate, which is why the f-stop series steps by the square root of two. EV also maps onto scene brightness. Reflected-light meters use EV equal to log2 of luminance times ISO divided by a calibration constant K, set at 12.5 by Canon and Nikon and 14 by Sekonic and Minolta, while incident meters use a constant C of roughly 250 to 340. The luminance and illuminance figures here are approximations on that basis. Useful anchors: EV 15 is direct sun, the condition behind the Sunny 16 rule of f/16 at one over the ISO; EV 12 to 13 is bright overcast; EV 7 to 8 a well lit interior; EV 5 or below usually needs a tripod. Meters assume a mid-grey reflectance near 18 percent and will misjudge snow or black subjects, and film suffers reciprocity failure beyond roughly one second.

The exposure value scale originated in Germany in the early 1950s, where it was known as the Lichtwert or light value. Friedrich Deckel's shutter works in Munich, maker of the Compur and Synchro-Compur leaf shutters, built the idea into hardware by coupling the aperture and shutter speed rings so that once a photographer set a single EV number the pair could be rotated together, sliding through equivalent exposures without changing the total light. Cameras of the period, including Rolleiflex twin-lens reflexes and Hasselblad bodies with Compur shutters, carried EV scales, and separate handheld meters were marked in the same units so a reading could be transferred directly. Standardisation followed on both sides of the Atlantic, in German DIN work and in the American standard ASA PH2.5-1960, which formalised the APEX framework, the Additive System of Photographic Exposure. APEX expressed aperture, time, sensitivity and brightness as separate logarithmic terms so that aperture value plus time value equals exposure value equals brightness value plus speed value, reducing exposure to addition at a time when this had to be done in the head or on a slide rule. APEX faded once through-the-lens metering and automatic exposure arrived in the 1960s and 1970s, but it did not disappear: the Exif metadata standard adopted in the mid-1990s stores ApertureValue, ShutterSpeedValue and BrightnessValue as APEX quantities, so the system introduced for mechanical shutters still travels inside digital image files.