How DSLRs work
You may know what a Digital SLR camera is, but how does it work? In the following simplified image, the relevant parts of the camera are colour-coded.
Digital SLR Camera
-Light passes through the lens and strikes a mirror (green)
-The mirror reflects the light up to a focusing screen
-Light passes through the focusing screen and enters a block of glass called a pentaprism (orange)
-The pentaprism reflects the image so that it can be seen in the viewfinder
-When a photo is taken, the mirror flips up and a shutter (blue) opens that exposes the digital sensor (red) to light.
This means that what you see in the viewfinder is what will be captured in the photograph, once taken. By using the viewfinder you can precisely compose your image and adjust the focus, in order to produce the most accurate representation. However, this does not mean that the viewfinder image and the captured image will be 100% identical.
Many digital SLR viewfinders will only show 95% of the image that is to be captured by the sensor - this is referred to as ‘coverage’. A sub-100% coverage is common, and will be quoted in many reviews. In practice, unless you are extremely precise when it comes to your photographs, the 5% difference is negligible.