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Redmoon Converters
📷 Photography

Exposure Triangle Calculator (ISO, Shutter, Aperture)

Calculate equivalent exposures across aperture, shutter speed and ISO. Each click is exactly one stop, with fractional stop support and EV table output.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

Exposure value (EV at ISO 100)
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Δ from base: 0 stops · Last locked: none
If you change…To match exposure, adjust…
Quick reference

Each "stop" is a doubling or halving of light. Doubling ISO, opening the aperture by one stop, or doubling shutter time all add one stop. Equivalent exposures keep ISO_stops + aperture_stops + shutter_stops constant.

EV at ISO 100 = log₂(N² / t), where N = aperture and t = shutter in seconds. Change any one setting and the tool moves another to hold the exposure: ISO is what moves by default, then aperture, then shutter — lock an axis to take it out of that order. ISO and aperture step in thirds where cameras do; the shutter ladder here is whole stops only, and the ISO ladder goes to whole stops above 25600.

How it works

The headline figure is exposure value at ISO 100: EV = log₂(N² ÷ t), where N is the f-number and t is shutter time in seconds. The default f/8 at 1/125 s gives EV 12.96. Underneath, the Δ from base readout counts stops of brightness relative to the ISO 100 · f/8 · 1/125 reference by summing log₂(ISO ÷ 100), log₂(64 ÷ N²) and log₂(t × 125) into a single signed number.

The three dropdowns are ladders of genuine camera values: ISO 50 through 102400 and apertures f/0.95 through f/32 in third-stop increments, shutter speeds 30 s down to 1/8000 in whole stops. Changing one axis moves another to hold the exposure matched, and the Lock ISO, Lock Aperture and Lock Shutter buttons take that axis out of contention — click again to release. Reset (ISO 100 · f/8 · 1/125) returns everything to the reference.

Check the compensating value against your camera before you shoot. The tool snaps to the nearest ladder entry, and because shutter is a whole-stop list while ISO and aperture are thirds, a third-stop change can round away to nothing on the shutter axis. It is also pure light arithmetic: no reciprocity failure, no ND filters, no T-stop transmission losses, and no accounting for the depth of field and diffraction that separate f/1.4 from f/16.

Frequently asked questions

What does EV 12.96 at ISO 100 actually mean?

It is log₂(N² ÷ t) for the default f/8 at 1/125 second — a measure of how much scene light that combination is set for, roughly bright overcast daylight. Higher EV means a brighter scene, and each whole number is one full stop.

If I open the aperture two stops, how much faster must the shutter be?

Four times faster. Each stop doubles the light, so two stops is a factor of four: f/8 at 1/125 becomes f/4 at 1/500. ISO could absorb the same change instead, dropping from 400 down to 100 for an identical result.

Why are shutter speeds whole stops when ISO and aperture use thirds?

The shutter list follows the classic camera dial, 30 s down to 1/8000 with each entry double the last. ISO and aperture use third-stop ladders instead. A one-third change on either can therefore round back to the same shutter value.

What do the lock buttons do?

They pin one axis so it will not be moved when you change another, and clicking a second time releases it. Use Lock ISO when your camera sits at a base ISO you want to keep, or Lock Shutter to protect a motion-freezing speed.

Is ISO 200 at f/8 the same exposure as ISO 100 at f/5.6?

Yes, at the same shutter speed. Doubling ISO adds a stop and opening f/8 to f/5.6 adds another, so both combinations land at the same brightness. They differ in noise and depth of field, neither of which this calculator models.

Three controls, one exposure, three side effects

Aperture, shutter speed and ISO each change exposure by the same amount per stop, which is what makes them trade freely. What does not trade is what else each one does: aperture sets depth of field, shutter speed sets motion rendering, and ISO sets noise.

That is the whole craft of the triangle — you choose which side effect you want and let the other two settle to reach correct exposure. A stop is a doubling or halving of light in every case, so the arithmetic is portable between them.

Where the equivalence breaks down

The three are equivalent for exposure and not for image quality. ISO is not simply a gain control on modern sensors — many have dual-gain designs where noise performance improves at a particular higher ISO, so the lowest ISO is not always the cleanest choice.

Aperture has limits at both ends: wide open, most lenses are softer and have more aberration; stopped well down, diffraction takes over. The sharpest aperture on a given lens is usually a couple of stops from wide open, and that is a lens property rather than a rule.

Shutter speed interacts with the subject rather than the sensor. There is no exposure substitute for a shutter speed fast enough to freeze what is moving, which is why it is usually the setting to fix first and let the others follow.

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