Shutter Speed and Exposure
You can change the time this curtain stays lifted. That’s what we call “shutter speed”. The longer the sensor is exposed, the more light will get in. Choosing a higher shutter speed, we double the amount of light that gets to the sensor.
Now you know what “long exposure” means. It’s a shutter speed setting at which the sensor stays exposed for a long time. Long exposures are used when there is not enough light for example at sunset times or during the night.
Controlling Motion With Shutter Speed
But, just like with aperture, things are not that simple. Shutter speed doesn’t just influence the amount of light that hits the sensor. It also changes the way motion is captured.
Higher shutter speeds such as 1/500 and above allow us to freeze the object. A running person will appear perfectly sharp. But if we choose a low shutter speed, his movement will look blurred in the final photo.
Here we can’t really say which exposure was “correct”. Sports photographers do need to freeze the action (we want to see which football player kicked the ball, right?) but blurred motion is often used deliberately to create a sense of dynamics or to make the final image more compelling.
A very good example, traditionally exploited by landscape photographers, is blurring water. Water shot at low shutter speeds acquires a very soft, milky look that is especially eye-pleasing. At such low speeds, any movement of the camera will blur the object in focus so the use of a tripod is obligatory.