This is a key part of the optical viewfinder system of a digital SLR. It’s a five-sided prism inside a housing on top of the camera that reflects the image captured by the lens and formed on the camera’s focusing screen so that it’s the right way up and the right way round for viewing through the camera’s viewfinder eyepiece. Some cheaper DSLRs use a less expensive ‘pentamirror’ design instead. It costs less to make but does have a slight effect on the size and quality of the viewfinder image.
While most cameras now let you compose your shots using an LCD screen on the back, there are lots of times when a regular viewfinder is still preferable. For many people it's more natural to put your eye to a viewfinder eyepiece than it is to hold the camera at arm's length. Viewfinders are also more useful in bright light, when glare often makes it hard to see what's being displayed on an LCD screen. Viewfinders come in different types. Some older cameras have 'direct vision' viewfinders, digital SLRs have optical through-the-lens viewfinders while mirrorless cameras use electronic viewfinders.
These are an option on both DSLRs and in electronic viewfinders. You can use the grid to make sure horizons are level and buildings are vertical – some grids confirm to the ‘rule of thirds’ to help you get a satisfying composition.
The percentage of the scene shown by the viewfinder. In better DSLRs you see 100% of the scene that will be captured, but in cheaper models it might only be 95-97%. That small difference can lead to objects showing at the edge of the frame that you hadn’t realised were there.
An older camera designed still used by celebrated German manufacturer Leica. The ‘rangefinder’ is used for focusing – as you turn the focus ring on the lens, a small mirror in the top of the camera rotates to line up a ‘ghost’ image with the main image in the viewfinder. When this ghost image lines up, your subject is in focus.
The viewfinder in a digital SLR is optical because it’s created by an image formed by the lens on a glass ‘focussing screen’. The direct vision viewfinders on some compact cameras are optical because you’re seeing the world through a set of lenses and not via a digital display.