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.