Exposure Triangle Calculator

Exposure value is a logarithmic bookkeeping device: EV equals log2 of the f-number squared divided by the shutter time in seconds, with a correction of log2(ISO/100) when sensitivity departs from the ISO 100 reference. Because it is base 2, one unit of EV is one stop, meaning a factor of two in the light recorded. Aperture contributes as the square of the f-number because the entrance pupil area scales with the square of its diameter, which is why the full-stop series runs f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 in steps of the square root of two rather than in equal increments. Shutter and ISO are linear, so each doubling or halving is exactly one stop. Any two settings that yield the same EV are equivalent exposures, and this calculator solves for the shutter speed or the ISO that restores the original EV after you change the aperture. Equivalent does not mean identical. The same EV reached at f/2 renders a shallow depth of field while f/16 renders deep focus; 1/1000 s freezes a sprinter while 1/15 s smears them; ISO 100 gives clean shadows while ISO 6400 raises read and shot noise and compresses dynamic range, typically by two to three stops on current cameras. Useful anchors are EV 15 for bright sun on an average subject, the sunny 16 rule of f/16 at one over the ISO, EV 10 to 12 for open shade, and EV 5 to 7 for domestic interiors. The main limitation is that EV describes only the camera settings and says nothing about scene luminance; reflected-light meters are calibrated to an 18 percent mid-grey, so snow or a black subject needs one to two stops of compensation.

Each leg of the triangle was standardised separately. The f-number as a ratio of focal length to entrance pupil diameter was promoted in the 1880s and settled into an international series by the early twentieth century. Shutter speeds converged on the doubling sequence over the same period. Film speed was the last to be pinned down: the German DIN scale appeared in 1934 and the American ASA scale in 1943, and the two were harmonised as the ISO standard in the 1970s, giving the arithmetic doubling scale of 100, 200, 400 still used for digital sensor gain. Exposure value itself was a mid-century German contribution, introduced to make setting a camera easier rather than harder. Manufacturers of the 1950s built EV numbers directly into leaf-shutter cameras with a coupling ring, sometimes called the light value system, so that once a photographer dialled in an EV the shutter and aperture rings moved together and any pair on that line gave the same exposure. The related APEX system, published in the American standard for photographic exposure computation in 1960, extended this into a full additive scheme of aperture, time, speed and brightness values. Through-the-lens metering and then automatic exposure modes made manual EV arithmetic optional from the 1960s onward. The teaching device of a three-cornered exposure triangle is much more recent, spread by photography instruction from the 1990s and 2000s, notably Bryan Petersons Understanding Exposure, as a way to explain the trade-offs that automation hides.