Digital cameras take photographs using an image
sensor that converts light to electronic signals. The distinguishing
feature of single-lens reflex (SLR) cameras is that the optical
viewfinder displays the view through the camera lens. The “single lens”
in the name “single-lens reflex” is a reference to this feature: a
single lens serves both for taking photographs and for the viewfinder.
The “reflex” portion refers to the reflection of light. DSLR Cameras
are equipped with mirrors that guide light from the lens into the
viewfinder by reflecting it upward, hence the term “reflex.” The light
that is reflected upward falls on the viewfinder focusing screen; after
passing through the screen, it then proceeds through a pentaprism or
pentamirror to the viewfinder eyepiece window. This allows the
photographer to view the image from the camera lens directly in the
viewfinder. When the shutter-release button is pressed all the way
down, the mirror is raised and the light coming through the lens
proceeds straight through to the shutter curtain. The curtain
simultaneously opens to allow the light to fall on the image sensor and a
photograph is taken. The shutter then closes and the mirror drops back
into its original position. By linking the action of the shutter with
the movement of the mirror, the light passing through the lens can be
made to fall on either the viewfinder focusing screen or the image
sensor.
Some newer DSLR Cameras can display the view through
the lens in the monitor: this is known as “live view.” The mirror is
raised during live view; blocking the light that would otherwise reach
the focusing screen and preventing photographs from being framed in the
viewfinder.
