In photography, shutter speed, also called exposure time, is the length of time the digital sensor (or film in an older camera) is exposed to light while taking a picture.

In a DSLR, a physical shutter moves out of the way to let light fall on the camera's sensor which records the actual image. Think of it like opening and closing the curtains in your living room. When the curtains are closed, no light gets through. As soon as you open the curtains, it comes rushing in.

RELATED: What Are Mirrorless Cameras, and Are They Better than Normal DSLRs?

Mirrorless and smartphone cameras, which are getting more and more popular, don't have a physical shutter; instead, the sensor is always exposed to light. If you look at your smartphone, the sensor sits just behind the lens. There's no room for a physical shutter! When you take a photo with one of these cameras, the sensor gets turned on. It saves the light that hits it during the time a physical shutter would be open on a DSLR, and then turns off again.

How Shutter Speed Works

When you take a photo, the longer the shutter is open (or the sensor is activated), the more light hits the sensor. The more light hits the sensor, the brighter the image will be. Imagine you're filling a glass of water from a tap. If you leave the tap on for half a second, there'll only be a small splash of water in the bottom. If you leave the tap on for five seconds, it will probably fill right up.

Shutter speeds can range from the very quick—around 1/8000th of a second for some sports photography—to the very slow—upwards of 30 seconds for long exposure photos. Most shutter speeds that you'll use, however, fall somewhere in the middle.

If you take a photo with your smartphone in automatic mode (where you let it do all the work), it will try and use a shutter speed of between about 1/30th of a second and 1/500th of a second most of the time. What value it chooses depends on how much light there is.

Going back to the glass example: on a bright day, it's like you have a tap that with a really fast flow; water pumps out at an incredible rate. At night, you have a tap that only dribbles out a few drops; to fill the same glass, you need to hold it under the tap for a lot longer.

In photography, you want to make sure the glass gets filled up, but doesn't overflow. If you don't let enough light hit the sensor, everything will just look murky and black. If you let too much hit the sensor, you've got the opposite problem: everything just looks white.

What Shutter Speed Should You Use?

RELATED: Your Camera's Most Important Settings: Shutter Speed, Aperture, and ISO Explained

Shutter speed is really important in photography. It's one of the three settings that determine how your photos will look—but all those settings interact with each other, so in order to know what shutter speed you should use, you should also learn about those other settings. Check out our guide to your camera's most important settings for everything you need to know.

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