Before it reaches the camera’s sensor, the light has to travel through some basic camera structures. First, it has to pass through the lens elements, then the aperture and lastly the shutter. Lens elements can’t block the light. Because they are made of clear glasses. Aperture can block the flow of light partially by increasing and decreasing the diameter of the opening. Only Shutter can block the light flow completely.
What is shutter?
Shutter is like a time keeper or stop watch of a camera. It controls flow of light by opening & closing it. It follows the same principles of window shutter. When it is open it allows the the light to pass through it & no light can pass through a closed shutter.
It is the time between opening & closure of the shutter. shutter speed is expressed in seconds. It determines the length of time the shutter remains open to allow the light to pass through the lens aperture and expose the image sensor. For example a shutter speed of 1/30th of a second is slower than a shutter speed of 1/250th of a second. This is used in tandem with the ISO setting and the Aperture. When using a wider aperture a faster shutter speed is recommended to avoid over exposure.
A faster shutter speed is recommended when shooting fast action photography or even sports photography when it is intended to ‘freeze’ the action. A slower shutter speed is used when special effects such as motion blur, light trails or water blur is required to be shot.
Faster shutter speeds are suitable when the camera is hand held. Comparatively when the camera is mounted on a tripod a slower shutter speed is feasible and manageable. Creative photographers use slower shutter speeds along with a low ISO, wide aperture (in manual mode) and a Graduated neutral density filter to capture water trails or even motion blur in broad day light.
Read more Creative use of slow shutter speed