Which of the following best describes shutter speed?

Shutter speed refers specifically to the duration for which the camera's shutter remains open when taking a photograph. This interval of time plays a crucial role in how motion is captured; a fast shutter speed can freeze quick movements, while a slower shutter speed can create a sense of motion blur. Understanding shutter speed is essential for controlling exposure and influencing the visual dynamics of an image.

The other choices describe different aspects of photography but do not define shutter speed. The size of the aperture opening pertains to the aperture setting, which affects depth of field and exposure. The distance from the lens to the subject relates to focus and composition rather than the mechanics of the shutter. Lastly, overall brightness of the photograph is influenced by multiple factors including shutter speed, but it is not itself a description of shutter speed.