Shutter speed controls how long light, gathered by the lens, falls on the digital sensor (or film). Going back to our water analogy, it would be how long the water flows. Shutter speed is typically expressed in fractions of a second like 1/250 but can be much longer.

It would be impossible for someone to set completely still throughout a 5 second shutter opening. Moreover, few photographers can hold a camera still for more than 1/15th of a second. Even tiny movements by subject or photographer , will create blurriness on the final image. Often, a photographer wants a shutter speed of 1/60th of a second of faster when taking a portrait to prevent blurriness from tiny movements. With action shots like sports or busy toddlers and/or long telephoto shots, One needs a faster and faster shutter speed to stop the action. A shutter speed of 1/500 – 1/1000 is a common for sports.

Assume one wishes to capture a young child at play and, as a result, wants a shutter speed of 1/500th to freeze the action. Outside, on a bright day, the sun provides enough light that a narrow aperture and a small ISO setting can easily be used. But at night under lamp light, one would get a seriously underexposed image. It would be very dark or possibly totally black. In order to compensate for the lower ambient light in the scene, a wider aperture (larger hole) and / or increased ISO is required to make an image.