The calculator
Exposure value for your shot
Change any input and the results update instantly. EV is always shown at the ISO 100 reference; the ISO-normalized scene EV appears as a separate output when ISO differs from 100.
Aperture & shutter
The f-stop you are shooting at. Larger numbers admit less light.
Stored internally in seconds (e.g. 1/125 s = 0.008 s).
ISO sensitivity
ISO does not change EV — it shifts the scene brightness that EV correctly exposes.
EV =
How it works — the formulas in full
EV is a log-base-2 scale. Every increase of 1 EV halves the light admitted; every decrease of 1 EV doubles it — one photographic stop. The two formulas below are the only arithmetic involved.
Variable definitions
What each symbol means
N — the f-number (aperture). A dimensionless ratio; f/8 means N = 8. Light admitted is proportional to 1/N², so doubling N halves the area and quarters the light.
t — shutter time in seconds. Longer shutter = more light. 1/125 s → t = 0.008 s.
EV — exposure value at the ISO 100 reference. Defined solely by N and t. This is the extracted value from the source (aperture + shutter); ISO does not appear.
ISO — sensor sensitivity. Does not change EV. It changes the scene brightness that EV corresponds to: higher ISO lets the same EV correctly expose a darker scene.
Scene EV — EV normalized for ISO. Shows what scene brightness the camera is set to expose correctly at the chosen ISO. At ISO 100 the normalization term is zero and Scene EV equals EV.
Equations
EV = log₂( N² / t )
N is the f-number; t is shutter time in seconds. EV is defined at ISO 100. ISO does not appear. This is the value extracted directly from the aperture-and-shutter combination.
ISO shift = log₂( ISO / 100 )
At ISO 100 this is log₂(1) = 0 — no adjustment. Each time ISO doubles, the shift increases by 1 stop. This term is calculated from ISO; it is not part of the EV definition.
Scene EV = EV − log₂( ISO / 100 )
Describes the scene brightness the camera is calibrated to expose correctly at the chosen ISO. Raising ISO makes the sensor more sensitive, so the same aperture+shutter setting can correctly expose a scene that is one or more stops darker — Scene EV decreases while EV stays the same.
Worked example — step by step
f/8, shutter 1/125 s, ISO 100. These are the calculator's default inputs. Every figure below can be reproduced by hand or verified with a pocket calculator.
Inputs
f/8 · 1/125 s · ISO 100
N = 8 | t = 1/125 s = 0.008 s | ISO = 100
Step 1 — Square the f-number
N² = 8² = 64
This is proportional to how much the aperture restricts light relative to a fully open lens.
Step 2 — Divide by shutter time
N² / t = 64 / 0.008 = 8,000
This ratio encodes the total light admitted by the aperture-and-shutter combination.
A larger ratio means less total light (higher EV).
Step 3 — Apply log base 2
EV = log₂(8000)
log₂(8000) = ln(8000) / ln(2) = 8.9872 / 0.6931 = 12.97 ≈ 13.0
This is the standard EV, defined at ISO 100. It depends only on aperture and shutter.
Step 4 — ISO normalization term
ISO shift = log₂(100 / 100) = log₂(1) = 0.0 stops
At ISO 100 the normalization term is exactly zero. EV and Scene EV are the same.
Step 5 — Scene EV
Scene EV = EV − ISO shift = 13.0 − 0.0 = 13.0
At ISO 100, this combination correctly exposes a scene at EV 13 — a typical
bright overcast day or open shade in full sun.
Step 6 — Interpret the result
EV 13 means you would need to remove 13 doublings of light from a theoretical EV 0 baseline (f/1 at 1 s). Any other aperture-and-shutter combination that also gives N²/t = 8,000 shares this EV — for example f/5.6 at 1/250 s (5.6² / 0.004 = 7,840 ≈ 8,000) or f/11 at 1/60 s (121 / 0.0167 ≈ 7,246 ≈ EV 12.8). Those are equivalent exposures.
Common EV values and typical scenes
A reference for typical outdoor and indoor EV values at ISO 100. These are the scene brightnesses your meter is trying to match — they are not set on the camera. Bright daylight typically measures EV 13–15; indoor available light EV 5–7.
| EV (ISO 100) | Typical scene | Example combination |
|---|---|---|
| 15 | Bright sun on snow or sand | f/16, 1/125 s |
| 14 | Bright sunny day, direct sun | f/11, 1/125 s |
| 13 | Bright overcast / open shade | f/8, 1/125 s |
| 12 | Lightly overcast sky | f/5.6, 1/125 s |
| 10 | Deeply overcast / twilight | f/5.6, 1/30 s |
| 7 | Brightly lit indoor scene | f/2.8, 1/30 s |
| 5 | Typical home / office interior | f/1.8, 1/30 s |
| 2 | Candle-lit scene | f/1.4, 1/4 s |
| 0 | Baseline reference (very dark) | f/1, 1 s |
All EV values are at ISO 100. Scene values are approximate — actual illuminance varies with time of day, latitude, season, and atmosphere. Use these as starting points for manual exposure or to verify that your meter is reading in the expected range.
Common mistakes with exposure value
EV is a precise technical term that photographers frequently misuse. These are the errors that come up most often.
Thinking EV includes ISO
The EV formula is EV = log₂(N²/t). ISO does not appear. EV is defined at ISO 100 and describes the aperture-and-shutter combination only. ISO determines what scene brightness that EV correctly exposes — it does not change the EV number itself. This is the single most common misunderstanding: a photographer who says "I raised EV by shooting at ISO 1600" has confused EV with effective exposure.
Confusing EV with exposure compensation
EV is an absolute value describing a specific aperture-and-shutter combination. Exposure compensation (the ±EV dial on your camera) is a relative offset — it asks the camera to shift its metered result by that many stops. "EV 13" and "+1 EV compensation" are completely different things. The camera's EV compensation control borrows the EV unit for its scale but does not set an absolute EV.
Forgetting that a smaller f-number means lower EV, not higher
Because EV = log₂(N²/t), a smaller f-number (wider aperture, more light) produces a smaller N², which gives a lower EV. f/1.4 at 1/125 s gives EV = log₂(1.96/0.008) ≈ 7.9 — much lower than f/8 at 1/125 s (EV ≈ 13). Lower EV means more light. Photographers used to thinking "higher f-number = more depth of field = better" sometimes flip this intuitively.
Mixing up the EV scale direction with camera metering
Your camera's meter aims to make the scene EV match the chosen ISO. If the scene is brighter than EV 13 at ISO 100, the meter will choose a faster shutter or smaller aperture to bring EV up to match — not down. The direction of change is the opposite of what feels intuitive: a brighter scene produces a higher EV reading, not a lower one.
Treating EV stops as equal to exposure compensation stops in all contexts
One stop of EV change is a factor of 2 in N²/t — the product of aperture area and shutter time. One stop of ISO is also a factor of 2 in sensor gain. They are numerically equal and interchangeable for exposure purposes — but they are physically different: EV changes affect optical light admitted; ISO changes affect electronic amplification of the signal. Confusing them matters for noise analysis: raising ISO by 1 stop does not give the same signal-to-noise ratio as opening the aperture by 1 stop or doubling shutter time.