Exposure Value and the Photographic Stop
Before pushing ahead to discuss how your camera’s settings affect exposure and picture brightness, it’s essential to familiarize yourself with the concept of exposure value, or EV. In photography, exposure value has two related but separate meanings: first, as a scale of absolute values, and second, as a unit of relative change.
Exposure value as an absolute scale
An exposure value is a number on a scale that represents scene luminance, which is the amount of light present in your composition and falling on your subject (be it a person, place, or thing). Each number on the scale is derived from all possible combinations of a camera’s aperture and shutter values that produce equivalent exposures at ISO 100.
The mathematical equation for how exposure values are derived is too advanced for a beginner’s guide and unnecessary for understanding correct exposure. Attempting to memorize the table below is also discouraged because you would need a second chart to convert the exposure values into aperture and shutter settings.
There are two reasons for introducing the scale. First, camera manufacturers use absolute exposure values to indicate the operating limits of their cameras’ autofocus and exposure metering modules. High-performance cameras tend to have a broader operating range. Second, the exposure value scale demonstrates the immense differences in the intensities of light you and your camera encounter across a variety of scenarios. How’s this possible for an apparently modest range of EV –6 to EV 18? The scale is logarithmic.
Exposure Values Scale
| EV (100) | Lighting Condition |
|---|---|
| >16 | Rarely encountered artificial sources of illumination. |
| 16 | Light-coloured surface, such as snow or sand, in direct midday sunlight |
| 15 | Direct midday sunlight. Full moon at night, high in sky. |
| 14 | Hazy sunlight or partly cloudy conditions. |
| 13 | Midday, cloudy/overcast |
| 12 | Subjects in open shade during clear midday sunlight. Skyline during sunset. |
| 11 | Sunsets. Subjects in deep shade. |
| 10 | Art gallery interiors. Land- and cityscapes immediately following sunset. |
| 9 | Floodlit sporting events. Art gallery interiors. Neon lights. Fires. |
| 8 | Bright street lights. Office interiors. |
| 7 | Indoor sports. Stage shows. Shaded forest floors during daytime. Light outdoors about 10 minutes after sunset. |
| 6 | Brightly lit home interiors. |
| 5 | Night home interiors. Night car traffic. |
| 4 | Candlelight. Christmas tree lights. Home interiors. |
| 3 | Monuments, statues, and fountains lit by floodlights. |
| 2 | Distant office buildings at night. |
| 1 | Distant skylines at night. |
| 0 | |
| –1 | |
| –2 | Subjects under full moon on snow or sand. |
| –3 | Subjects under full moon. |
| –4 | Subject lit by gibbous moon. |
| –5 | Subjects lit by starlight and crescent moon. |
| –6 | Subject lit by starlight. |
Exposure value as units of relative change
An exposure value also describes an interval on the scale above. Adding 1 EV corresponds to double the light intensity; subtracting 1 EV corresponds to half the light intensity. This relationship makes the scale logarithmic. For example, EV 15 indicates a subject brightness half that of EV 16 (–1 EV difference, or 1/(2^1) = 1/2) and sixteen times that of EV 11 (4 EV difference, or 2^4 = 16).
The photographic “stop”
Beyond its utility in describing the differences in absolute scene luminance, relative EV is used to describe a specific change to exposure. In this role, it’s commonly called a stop, or less often, a step.
A stop is a unit of change to the relative aperture or shutter settings that either doubles or halves the exposure. When referring to ISO, a stop either doubles or halves the effective exposure, meaning the resulting picture brightness.
Does 1 EV refer to a specific position on the absolute scale or a relative unit of change? No standardized notation exists for differentiating between the two. When in doubt, use context to determine the author’s intent. This guide refers to the absolute scale using EV # and to the relative scale using # EV.
Sometimes, it’s not possible to achieve correct exposure by changing settings using full stops. In such circumstances, photographers rely on fractions of a stop. Most cameras allow adjusting exposure settings using one-half and one-third stop intervals, and some professional flash units enable adjustments as fine as 1/10-stop intervals.
It’s essential to understand that a stop has no absolute value; it always describes a change corresponding to an existing amount.