In photography, a rangefinder is a generic term for a type of camera. Rangefinder cameras do not view and focus through the lens like an SLR camera. Instead, they focus using a dual-image mechanism whereby the scene is viewed through a special ‘range-finding’ window. Most rangefinder cameras show these two images of the same subject and the photographer turns the focus ring until the two superimposed images line up, at which point you’ve achieved focus. It’s been many decades since Canon dropped the rangefinder format in favour of SLR.