A single lens reflex camera (SLR) uses a pentaprism and an angled mirror to redirect light up from the lens to the viewfinder, allowing the photographer to see precisely the image that the film will record. The word reflex refers to the mirror reflection and is derived from the German word reflexion. A lens will invert light both horizontally and vertically, so the mirror and pentaprism reinvert the image in the viewfinder, making it appear exactly as it is seen with the naked eye. These optics are housed in a mirror chamber between the rear of the lens and the focal plane and directly above this in the top of the camera body. This chamber takes up considerable space, increasing the size of the camera.
When the shutter of an SLR is released, the mirror moves up out of the light path so the image can be directed onto the film. This causes the viewfinder to black out when you take a picture (mirror blackout), making it impossible to see the exact moment you capture on film. (The mirror isn’t in place to redirect the image into the viewfinder.) The movement of the mirror is called a slap. This is relatively noisy and causes the camera to shake, which can make an image blur at slower shutter speeds.
Unlike an SLR, a rangefinder camera does not allow the photographer to look directly through the lens. Images are instead previewed through a viewfinder window usually found on the top right corner of the camera body when viewed from the front. (Assuming a dominant right eye, this is more comfortable for shooting with both eyes open.) A rangefinder’s simpler optical design requires less space than the pentaprism and mirror chamber of an SLR, and although the separation of viewfinder and lens can cause problems with framing the image (see parallax), the most noticeable benefit is in the size of the camera. Rangefinders are much smaller and lighter than most SLRs and hence more portable. The lack of mirror chamber also improves the quality of image, as the lens can be mounted deeper into the camera body, shortening the distance between the rear lens element and the film. This means that the image can be projected from lens to film with minimum loss of color and detail, since light travels a much shorter path.
With no moving mirror, a rangefinder doesn’t suffer from mirror slap, and the radial motion of a fixed-lens rangefinder’s leaf shutter induces less vibration than the lateral motion of a focal-plane shutter. This not only makes it quieter than an SLR, it makes it less susceptible to motion blur caused by camera shake, allowing the photographer to achieve sharp images with a handheld camera at slower shutter speeds. This also means that there is no mirror blackout in a rangefinder, so you can witness the decisive moment when the shutter is released: you are able to see if your subject moved and can be sure that you captured the image you were aiming for.
Focal-plane shutters on SLRs (and interchangeable-lens rangefinders) are capable of much faster speeds than leaf shutters but cannot sync with a flash at all speeds. Because SLRs don’t have to compensate for parallax, they are generally capable of focusing much closer than rangefinders. The closest focusing distance on a rangefinder is around 3 feet, whereas an SLR with equivalent lens will be able to focus at least 1 foot closer. However, it’s much quicker to focus a rangefinder lens from the closest distance to infinity than it is with the equivalent SLR lens.
