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Think of an SLR like a periscope – it employs an angled, reflex mirror and a pentaprism to redirect light up to the viewfinder in order to allow the photographer to see precisely the image that the film will record. These optics are housed in a mirror chamber between the rear of the lens and the film plane, and take up considerable room which in turn increases the size of the camera.
When the shutter is released, the mirror moves out of the light path so the image can be directed at the film. This causes the viewfinder to blackout when you take a picture (mirror blackout) making it impossible to see the exact moment you capture on film. The movement of the mirror is comparable to a slap; it is not only noisy but also causes the camera to shake which can cause the image to blur at slower speeds.
Unlike an SLR, a rangefinder camera does not employ a system of mirrors to enable the photographer to view directly through the lens. It instead uses a viewfinder that is not connected to the lens. Although this can cause problems with image framing (see parallax) for us, the pros far outweigh the con. The most noticeable benefit of not using mirrors is seen in the size of the camera. Rangefinders are much smaller and lighter than most SLRs making them more portable. The lack of mirror chamber also improves the quality of image recorded as the lens can be mounted deeper into the camera body thus shortening the distance between the rear lens element and film. This means that the image can be projected from the lens to film with minimum loss of color and detail, since light travels through a much shorter path.
With no moving mirror a rangefinder doesn’t suffer from mirror slap. The radial motion of a 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 hand held 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.