Exposure Value Explained: How Aperture and Shutter Combine Into One EV Number
A working photographer's guide to exposure value (EV): the log2 formula, stops, reciprocal settings that share one EV, and using EV to nail exposure in tricky light.
Exposure Value Explained: How Aperture and Shutter Combine Into One EV Number
The first time someone told me a scene was "EV 15," I nodded along and had no idea what they meant. Aperture I understood. Shutter speed I understood. But a single number that folded both of them together felt like a secret handshake. It turns out exposure value is one of the most practical ideas in photography once it clicks, and the math behind it is short enough to fit on a napkin.
This post walks through what EV actually is, the formula that produces it, why a dozen different aperture and shutter pairs can all land on the same number, and how I use EV in the field when the light is fighting me.
What Exposure Value Actually Measures
Exposure value is one number that captures how much light a particular aperture and shutter speed let reach the sensor. A bigger EV means a brighter scene needs less light gathering; a smaller EV means a darker scene needs more. The reading is defined at ISO 100, so when you see "EV 12" with no ISO mentioned, ISO 100 is assumed.
The unit that matters here is the stop. One full EV equals one stop, and one stop is a doubling or halving of light. EV 15 is one stop brighter than EV 14, which is one stop brighter than EV 13, and so on. That single fact, that +1 EV halves the light the camera needs to collect, is the spine of everything else.
A bright midday street meters near EV 15. Open shade is around EV 12. A dim indoor room might be EV 6 or 7. A moonlit landscape sits down near EV minus 3. Knowing roughly where a scene lands lets you sanity-check a meter, or work without one entirely.
The Formula: EV = log2(N²/t)
Here is the whole thing:
EV = log2(N² / t)
N is the f-number (the printed aperture, like 8 or 5.6), and t is the shutter time in seconds. Two pieces explain why it is shaped this way.
The aperture term is N squared because the opening's area scales with the square of its diameter. An f-number is a ratio, and a smaller number means a wider opening, so f/2.8 lets in far more light than f/8. Squaring N converts the diameter ratio into the area of light coming through.
Dividing by t accounts for how long the shutter stays open. A longer exposure gathers more light, so dividing by a larger t produces a smaller EV. Finally, log2 turns that ratio into stops, since each stop is a factor of two. The base-2 logarithm is exactly the "how many doublings" question in math form. If you want to see the log step on its own, the scientific calculator will take log2 directly.
Plug in f/8 at 1/250 s:
EV = log2(8² / (1/250)) = log2(64 × 250) = log2(16000) ≈ 13.97
So f/8 at 1/250 s is just shy of EV 14. That single value is the anchor for the next idea.
Why Many Settings Share One EV
Because EV is a ratio collapsed into stops, equivalent settings share one EV. Any aperture and shutter pair that produces the same N²/t produces the same brightness on the sensor. These are reciprocal exposures: give up a stop in one place, take it back in the other, and the exposure does not move.
Start from our anchor, f/8 at 1/250 s near EV 14. Open the aperture one stop to f/5.6. That doubles the light coming through, so to hold the same EV you must halve the time the shutter is open: 1/250 s becomes 1/500 s. Check it:
EV = log2(5.6² / (1/500)) = log2(31.36 × 500) = log2(15680) ≈ 13.94
Same EV, give or take rounding. f/8 at 1/250 s and f/5.6 at 1/500 s are one EV in practice, two settings that expose identically. You could keep going: f/4 at 1/1000 s, f/2.8 at 1/2000 s, all the way along the dial. Each step opens the aperture one stop and shortens the shutter one stop, and the brightness never changes.
What does change is everything else about the photo. The wider aperture throws the background out of focus; the faster shutter freezes motion harder. That is the real payoff of thinking in EV: it separates "how bright" from "how it looks," so you can lock the brightness and then choose your depth of field and motion freely. When you want to see what each of those reciprocal pairs does to focus, the depth of field calculator shows the trade in millimeters.
Stops, ISO, and the Third Leg
Aperture and shutter are two legs of the exposure triangle. ISO is the third. The base formula assumes ISO 100, and above that you subtract log2(ISO/100) from the reading. Doubling ISO from 100 to 200 subtracts log2(2) = 1, so the metered EV drops by exactly one stop, because the sensor now needs one stop less light. ISO 400 drops two stops, ISO 800 drops three.
Stops are the common currency across all three controls, which is what makes them interchangeable. A one-stop change anywhere can be undone by a one-stop change somewhere else. Open the aperture one stop and you can shorten the shutter one stop, or drop the ISO one stop, and the picture's brightness holds. Counting in stops instead of raw numbers is the habit that makes fast exposure decisions possible.
The whole-stop f-number series is worth memorizing: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Each step halves the light. Shutter speeds run in a parallel series: 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, each step also halving the light. Once those two ladders live in your head, sliding up one and down the other is automatic.
Using EV in Tricky Light
The reason I keep an exposure value calculator in my workflow is that field light rarely cooperates. A few situations where EV earns its keep:
No meter, bright sun. The Sunny 16 rule says direct sun is about EV 15: set f/16 and a shutter near 1 over the ISO. At ISO 100 that is f/16 at roughly 1/125 s. Confirm it reads near EV 15, then swap to any equivalent pair the moment calls for. Want a blurred background? Slide to f/4 at 1/2000 s. Same exposure, dreamy separation.
Fading light at the end of a shoot. Your shutter is already at the slowest speed you can hand-hold. Note the current EV, then bump ISO and watch the metered EV fall by whole stops. Going to ISO 800 drops it three stops, and that number tells you precisely how much brightness you are buying back with sensitivity before grain becomes the only lever left.
Double-checking a long exposure. Before burning a multi-second night frame, I punch the settings in and compare the EV against where a night scene should land. If f/11 at 8 s and ISO 200 reads wildly off the expected range, I caught a dial error before wasting the shot.
The deeper habit EV teaches is to decide brightness once and then spend your stops deliberately. Lock the EV, and every other choice becomes a trade you control rather than a guess you hope works out.
If you want to push the numbers further, working out how many stops separate two settings is just a ratio in disguise, and the percentage calculator is handy when you are reasoning about exposure changes as fractions of the light. But most days, the only number I really need is the EV, and a clear sense of which way the stops run.
Made by Toolora · Updated 2026-06-13