What is a "Stop" of Exposure in Photography?

Varvara May 05 2022

"Stop" is a term from photography that gets thrown around a lot. Someone describes a photo as an underexposed stop or asks you to increase the shutter speed by one stop. The concept can be a bit confusing for new photographers. So let's take a close look at what a stop is and what it means for photography.

The term stop in photography means “a relative change in the brightness of the light”. It's important to understand that this isn't just limited to aperture and shutter speed, but also to the film itself. Film can capture or absorb light, but the concept is the same. The three factors interact in a 'trinity'.

What is a stop in exposure?

The concept of a "stop" is widely misunderstood in photography, and many people are afraid of it because it sounds so complex. The concept is simple, however.

When you take a photo, exposure is determined by the range of the aperture and exposure time (also known as shutter speed). Although the exposure is basically quantityless, there are a number of combinations of aperture and shutter speed that will create a good photographic exposure. 

If the aperture is too large or the exposure time is too long, you will only get a white photo. Conversely, if any of them are too low, you will only get a black photo. One stop is equivalent to halving (or doubling) the amount of light let into the camera by that factor.

How to calculate the stops

For example, if you set your camera's shutter speed to 1/100 second, increasing the exposure by one stop will change the shutter speed to 1/50 second (allowing twice as much light to enter the camera). Changing the shutter speed to 1/200th of a second (halving the amount of light let into the camera) reduces the exposure by one stop. 

As you can probably see, the shutter speed rule is really simple: to increase exposure by one stop, halve the shutter speed. Double exposure to decrease exposure.

Intermediate exposure stops in photography

Photographers also speak of intermediate stops or third stops. Third stops are particularly important as they are the increment most cameras use for their shots. These are just imaginary divisions in each stop. 

To slacken your shutter speed by a third of a stop, reduce it by a third of what it takes to slacken it by one point. Continuing with the example above to decrease the shutter speed from 1/100 second by a third of the stop, change it to around 1/80 second.

Aperture stops in photography 

With the aperture, things are much more complicated. When we say we're using an f/10 aperture, it means the diameter of the aperture is equal to the focal length of the lens divided by 10. If we use a 100mm lens, this gives a diameter of 10mm. 

The amount of light let into the lens through the aperture does not depend directly on the diameter, but on the area: this is calculated using πr², where r is the radius. This means that the ratios are much more difficult to calculate in your head. Closing the aperture down to f/20 doesn't cut the range in half, it roughly quarters it.

The third exposure factor, ISO, is also measured in stops. As with shutter speed, the relationship between the values ​​is simple. To increase your ISO by one stop, double the value, for example from ISO 100 to ISO 200. To decrease it by one stop, for example, halve it from ISO 1600 to ISO 800.

Stops in photography are approximate

There are two things to note about stops: first, the values ​​on your camera are approximate, and second, if the values ​​are extreme, there are other factors at play.

When you change your camera's setting, you only adjust it by about a third. For example, my camera's shutter speed ranges from 1/100 second to 1/80 second. That's a little over a third of a pause (it should be about 1/83 of a second). This discrepancy doesn't matter in the real world, but it's worth knowing it exists.

When you're working with extremely slow or extremely fast shutter speeds, other factors come into play. If you're capturing a 30 minute exposure in a very dark room, doubling the shutter speed to 60 minutes doesn't automatically make everything twice as bright. For most people, it doesn't matter. When you're working with extremely slow or fast shutter speeds, things aren't so clear cut.

When you start film photography, you might be surprised to hear 'stop' all the time. No, no one is mean to you, that's a fundamental concept in photography. Now that you understand what stop means in photography, you can apply it to your own craft. If a photo looks a little too dark, you need to up one of your exposure settings by a notch, or, if you've already taken the photo, lighten the exposure by a notch in Lightroom.