Exposure value, abbreviated EV, is a number that expresses all combinations of aperture and shutter speed that admit the same amount of light to the sensor or film. In photography, it is defined as EV = log2(N²/t), where N is the f-number and t is the shutter time in seconds, so f/1 at 1 second equals EV 0.
Because the scale is logarithmic in base two, one EV step equals one stop: a doubling or halving of light. This makes exposure value the common unit in which meters, cameras and photographers describe brightness, exposure compensation and dynamic range.
How exposure value is calculated
The formula combines the two settings that control how much light reaches the sensor. Squaring the f-number reflects that light passing through the aperture depends on its area, and dividing by the shutter time accounts for how long the light is collected.
Each increase of one EV means half as much light. Opening the aperture by one stop while making the shutter twice as fast leaves EV unchanged, which is the principle of reciprocity. The table shows equivalent settings for one exposure value.
| Aperture | Shutter speed | Typical reason to choose it |
|---|---|---|
| f/2 | 1/1000 s | Shallow depth of field, freezing motion |
| f/2.8 | 1/500 s | Portraits with soft background |
| f/4 | 1/250 s | General handheld shooting |
| f/5.6 | 1/125 s | Small groups, moderate depth of field |
| f/8 | 1/60 s | Landscapes with good sharpness |
| f/16 | 1/15 s | Deep depth of field on a tripod |
Real cameras use nominal f-numbers and shutter speeds that are rounded, such as 1/125 s instead of 1/128 s. The calculated EV values are therefore approximately whole numbers, which is accurate enough for practical work.
Exposure value, light value and ISO
Strictly, exposure value describes camera settings only. ISO is not part of the formula. When photographers say a scene “is EV 15”, they usually mean the EV that produces a correct exposure at ISO 100, a quantity also called light value (LV) or EV100.
Raising ISO by one stop allows a correct exposure at one EV higher, that is, with half the light. The relation is EV at a given ISO = EV100 + log2(ISO/100). This is how the exposure triangle connects to the EV scale.
The table lists approximate EV100 values for typical scenes. Real values vary with season, latitude, weather and subject tone, so they are starting points, not measurements.
| Scene | Approximate EV100 | Example setting |
|---|---|---|
| Bright sun on snow or sand | 16 | f/16, 1/250 s |
| Bright sunny day | 15 | f/16, 1/125 s |
| Overcast daylight | 12 to 13 | f/8, 1/60 s |
| Open shade or heavy overcast | 11 to 12 | f/5.6, 1/125 s |
| Bright office interior | 7 to 8 | f/2.8, 1/30 s |
| Home interior at night | 5 to 6 | f/2, 1/15 s |
| Floodlit buildings | 2 to 4 | f/2.8, 1 s |
| Full moon landscape | about -2 to -3 | f/2.8, 30 s or longer |
The bright-sun row is the basis of the classic sunny 16 rule: at f/16, set the shutter speed to the reciprocal of the ISO.
Where exposure value is used in practice
EV appears in many camera functions, always as a measure of stops of light. Understanding the shared unit makes these functions easier to use together.
| Function | Typical range | What the number means |
|---|---|---|
| Exposure compensation | ±3 to ±5 EV in 1/3 steps | Deviation from the meter’s suggested exposure |
| Exposure bracketing | Steps of 0.3 to 3 EV | Spacing between frames in a sequence |
| Metering range | Often about -3 to 20 EV at ISO 100 | Brightness range the meter can measure |
| Autofocus sensitivity | Down to about -4 to -7 EV on recent bodies | Darkest light in which focus still works |
| Dynamic range | About 10 to 15 stops for modern sensors | Span between noise floor and clipping |
| ND filter strength | 1 to 10 stops or more | Reduction of light reaching the lens |
Note the sign convention. A higher EV number in the settings sense means less light admitted, yet exposure compensation of +1 EV means a brighter image. The two use the same unit but opposite directions, which confuses many beginners.
Exposure value compared with related exposure concepts
Exposure value is the unit, while neighboring concepts describe controls or techniques that act on it.
| Term | What it is | Relation to EV |
|---|---|---|
| Aperture | Size of the lens opening as an f-number | One input of the EV formula |
| Shutter speed | Duration of the exposure | The other input of the EV formula |
| ISO | Sensor signal amplification | Shifts which EV gives correct exposure |
| Exposure compensation | Deliberate bias of the meter result | Expressed in EV steps |
| Spot metering | Meter reading from a small area | Returns an EV for that area |
Common misunderstandings about exposure value
Several misunderstandings appear repeatedly in practice. Most come from mixing the settings definition with the scene brightness usage.
- EV is not image brightness: the same EV produces different results at different ISO values.
- Higher EV settings mean less light, which is the opposite of how a brighter scene is described.
- A meter’s EV assumes middle gray, so snow or black subjects need compensation.
- Reciprocity can fail with film at very long exposures, where extra time is needed beyond the calculation.
- Equivalent settings are not equivalent images: depth of field and motion blur change even when EV stays constant.
A plain-language summary
Exposure value is one number that sums up how much light a camera lets in. Many different combinations of lens opening and shutter time give the same amount of light, and they all share the same EV.
Think of filling a bucket with water. A wide tap for a short time and a narrow tap for a long time can fill the bucket to the same level. EV describes the level, not how you got there. Each step on the scale doubles or halves the light, which makes it a simple way to talk about brighter or darker.
An example from practice
A landscape photographer at a lake on an overcast afternoon meters the scene at about EV 12 at ISO 100, which the camera suggests as f/5.6 at 1/125 s. She wants a smooth, blurred water surface and maximum depth of field, so she plans f/11 and a long exposure.
Moving from f/5.6 to f/11 is two stops, so the shutter time becomes 1/30 s. To reach about 2 seconds, she needs six more stops of light reduction and fits a six-stop ND filter: 1/30 s becomes about 2 s while the effective exposure stays at the metered level.
Because the snow on distant hills is bright, she also applies +0.7 EV exposure compensation and brackets three frames at 1 EV spacing. The middle frame holds detail in the snow and in the dark rocks, and the water shows the smooth texture she planned.