“Stop” is a photography term that gets thrown around a lot. Someone will describe a photo as a stop under-exposed, or tell you to increase your shutter speed by a stop. The concept can be a little confusing for new photographers, so let’s look at exactly what a stop is and what it means when it comes to photography.
Stops, Shutter Speed and Aperture
When you take a photograph, the exposure is determined by the area of the aperture and the exposure time (also called shutter speed). Although exposure is basically quantity-less, there are a range of combinations of aperture and exposure time that will create a good photographic exposure. If the aperture is too wide or the exposure time too long, then all you’ll get is a white photo; conversely, if either of them is too low, you’ll just get a black photo.
Since exposure is valueless—you don’t look at a scene and describe it as a 12 stop photo for example—there is no way to talk about things in absolutes. Instead, stops are used to describe relative changes in aperture and exposure time. One stop is equal to a halving (or a doubling) of the amount of light let into the camera by that factor.
So for example, if you have the shutter speed on your camera set to 1/100th of a second, increasing your exposure by one stop would change the shutter speed to 1/50th of a second (letting twice as much light into the camera). Changing your shutter speed to 1/200th of a second (halving the amount of light let into the camera) reduces your exposure by a stop. As you can probably see, for shutter speed the rule is really simple: to increase your exposure by a stop, double your shutter speed; to decrease your exposure by a stop, half it.
Photographers also talk about half-stops or third-stops. Third-stops are especially important as they’re the increment that most cameras use for their settings. These are…