Exposure Intervals in Digital Photography
Various combinations of camera settings create different exposures. They’re compared to each other in terms of stops and exposure values (EV). (You may be familiar with the term EV if you’ve ever shot exposure brackets for HDR or dialed in exposure or flash compensation. That’s because all those methods use EV to quantify and compare exposures.)
Stops are a traditional way of describing exposure intervals. They are mechanical. The term comes from how photographers changed apertures and shutter speeds on their film cameras. They would widen the aperture by a physical stop on the lens or make the shutter speed faster by turning a knob to the next stop. This doubled or halved the amount of light that the film was exposed to. The term therefore made its way into the lexicon of photography as a way to double or halve light.
Exposure value is a number that represents a given amount of light. You can change the EV by altering the camera’s exposure settings (aperture, shutters speed, or ISO) however you like. Each EV is the numerical equivalent of a mechanical stop. For example, raising the ISO by a stop increases the EV by +1.
The following figure shows the relationship between newer camera settings and stops. Each numbered division represents a stop of exposure, as on the older dial. Each stop is divided into thirds by smaller, unnumbered marks. Each stop is also a whole EV.
The standard exposure is placed at 0 EV on a scale representing possible exposures. This position indicates the combination of exposure settings that will create a properly exposed photo. Doubling the light creates an interval of +1.0 EV. Halving the light creates an interval of -1.0 EV. This interval is known as an exposure value. Exposure values are most often measured in thirds and whole numbers. You can often set your camera to control exposure level increments in halves.
Stops and EV are so powerful because you can exchange units of exposure without worrying where they came from. A stop is a stop, whether it comes from changing the shutter speed, aperture, or ISO. In terms of exposure (but not necessarily creativity), it doesn’t matter if you get a 1.0 increase by opening up the aperture, slowing the shutter speed, or increasing the ISO.