What is the Shutter

The recording of an image is determined by how many Photons of light strike the cameras sensor.  The more photons that are recorded, the lighter the image becomes until it is pure white.  In the section "What is the Aperture", you learned that a variable hole in the lens increases or reduces the amount of light that is allowed through the lens.  The shutter is another method of controlling how much light reaches the camera Sensor.  In the days of film there were two types of shutters: the Focal Plane Shutter, and the Leaf Shutter.  The job of the shutter was to protect the film from light until the shutter release button was pressed, and then remain open for a set period of time until enough light

Leaf Shutter

reached the film for a proper exposure.  A short exposure time can have the effect of freezing the action of a race car for instance, or a long exposure can capture stars or fireworks as they trail through the sky.  The Shutter, Aperture, and ISO all balance each other.  If you increase the ISO you make the camera more sensitive to light, so you must make the aperture smaller or the Shutter speed faster or both.  The will be lessons ahead that will make this more clear.

Shutter speeds range from fractions of a second as high as 1/8000 of a second or even faster, to whole seconds.  There is also a "Bulb" setting which will hold the shutter open for as long as you depress the shutter button.  The term bulb came from flash bulbs.  Early cameras had no way to synchronize the shutter and the flash bulb, so you would manually press flash button after you opened the shutter. Shutter speeds are either twice as

Focal Plane Shutter

fast or half as fast as the setting before it.  If you open the aperture by stop, you must increase the shutter speed by one stop to compensate.

Newer electronic cameras may or may not have a mechanical shutter.  Since there is no film in a digital camera, there is no need to worry about the sensor being exposed to light.  

When you depress the shutter button, voltage is applied to the sensor and each pixel is recorded to memory one row at a time.  Because the shutter traverses across the focal plane, distortion of the image can occur if the subject is moving faster than the shutter like the propeller of an aircraft.  If the entire image could be captured at the same instant, this distortion would not occur, but the image is exposed either vertically or horizontally in a

Image by Robin Blanford