“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, halve your shutter speed; to decrease your exposure by a stop, double 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 just imaginary divisions in each stop. So, to decrease your shutter speed by a third of a stop, you reduce it by a third of the value necessary to decrease it by a full stop. Continuing with the example from above, to decrease the shutter speed of 1/100th of a second by a third of stop, you’d change it to around 1/80th of a second.
With aperture, things are a lot more complicated. When we say we’re using an aperture of f/10, that means the diameter of the aperture is equal to the focal length of the lens divided by ten. If we are using a 100mm lens, that would give us a diameter of 10mm. The amount of light let into the lens through the aperture doesn’t directly depend on the diameter, however, it depends on the area: that’s calculated using πr² where r is the radius. This means that the ratios are a lot harder to calculate in your head. Closing your aperture down to f/20 doesn’t halve the area, it roughly quarters it.
Above, I’ve created a chart of common aperture values in third-stops. These should correspond to the values you can dial in on your camera. The simplest way to change your aperture by a stop is just to move the aperture dial on your camera three clicks.
The third exposure factor, ISO, is also measured in stops. Like shutter speed, the relationship between the values is simple. To increase your ISO by a stop, double the value, say from ISO 100 to ISO 200. To decrease it by a stop, half it, say from ISO 1600 to ISO 800.
Stops Are Approximate
There are two things worth noting about stops: first, the values on your camera are approximate and second, that at extreme values, other factors come into play.
On your camera, when you change the setting you are only adjusting it by about a third of a stop. For example, my camera’s shutter speed goes from 1/100th of a second to 1/80th of a second. That’s a little over a third of a stop (it should be about 1/83rd of a second). This discrepancy doesn’t really matter in the real world, but it’s worth knowing it exists.
When you’re working with extremely long or extremely short shutter speeds, other factors start to come into play. If you shoot a 30 minute exposure in a very dark room, doubling your shutter speed to 60 minutes won’t automatically make everything twice as bright. For most people, this won’t matter. Just know that if you’re working with extremely long or short shutter speeds, things won’t be as clear cut.
Now that you’ve got an idea of what stops are, you should see how they apply to your photography. If a photo looks a little too dark, you know that you need to increase one of your exposure settings by one stop (or, if you’ve already taken the photo, brighten the exposure in Lightroom by one stop).