Cheat sheet: Shutter speed stops

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All but the most basic cameras offer control over shutter speed, and it's one of the key ways in which we determine how our images end up looking. 

But what exactly is is shutter speed? And what do photographers mean when they refer to one stop or two stops? Read on to find out what you need to know.

What is shutter speed?

Shutter speed forms one part of the exposure triangle, the other two parts being aperture and ISO. Whereas aperture controls how large the opening inside the lens is for light to pass through, and ISO how much light is required in order to create the image, shutter speed dictates how long the camera's sensor is exposed to light. The right balance of the three is what every photographer needs to consider if they're to achieve the image they expect.

Read more: Cheat sheet: Wide vs narrow aperture

The easiest way to control shutter speed is to turn the camera's mode dial (assuming it has one) to the the Shutter Priority setting. Depending on which camera you use, this will be marked Tv (which stands for Time Value) or simply S. Once in this mode, you simply turn a command dial on your camera and you should see a figure in the viewfinder and/or the LCD screen jump between values like 1/125, 1/250, 1/500 and so on – and this is your shutter speed.

1/125 is 1/125sec, which means that the shutter inside your camera will be open for one-hundred-and-twenty-fifth of a second. This may be a physical shutter or an electronic one, but the principle is the same and your camera's sensor is exposed to light for this period of time. 

Read more: Why do small apertures have large f-numbers?

If you then change the shutter speed to 1/250sec, you're allowing half as much light to get through to the sensor. Change it once more to 1/500sec and you're halving this again, and then another halving would be 1/1000sec, and so on. Each time you halve it, or go in reverse and double it, you're moving by what's known as one 'stop'. So, a shutter speed of 1/125sec is one stop brighter than 1/250sec, as you're letting in twice as much light.

As you should be able to see when you adjust this, there are other settings in between these. Most cameras tend to move in third-stop increments, so you end up going from 1/125sec to 1/250sec via 1/180sec and 1/200sec options.

If you ever use the Manual exposure mode, or exposure compensation, you may already be familiar with this idea of stops. Each dot or line on the exposure scale inside your viewfinder (or on your LCD's display) represents a third of a stop, and a more pronounced line indicates a full stop. 

This graphical user interface (GUI) from a Nikon camera shows a shutter speed of 1/100sec on the left-hand side. The exposure scale in the middle of the display has lines to show full-stop increments and dots in between these to show half-stop increments. 

This graphical user interface (GUI) from a Nikon camera shows a shutter speed of 1/100sec on the left-hand side. The exposure scale in the middle of the display has lines to show full-stop increments and dots in between these to show half-stop increments. 

Unless you're using exposure compensation, the marker underneath these shouldn't move in the Shutter Priority mode as your camera will automatically be adjusting the aperture to compensate for whatever changes you make. Aperture also moves in full-stop and third-stop increments, and as you change shutter speed, the aperture either adjusts automatically, or needs to be adjusted, in order to keep the exposure balanced. 

So, if you choose a very fast shutter speed, your aperture will need to be wider in order to let in the same amount of light as if you were using a slower shutter speed (assuming you have your ISO fixed to a certain one setting, rather than left to auto). 

If you want to know more about what kinds of shutter speeds are appropriate for different kinds of images, check out this cheat sheet here

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