What is a single lens reflex?

Answer 1

A single lens reflex (SLR), as its name implies, allows the photographer to both view and record an image through the same lens. The advantage the SLR gave photographers was "What You See Is What You Get." The image as seen in the viewfinder is the image that will be recorded on the film (in a film camera) or sensor in a digital camera, regardless of the focal length of the lens mounted. Prior to development of the SLR, most cameras had two view paths; one to some type of viewfinder, and the other to the film. This was not a significant problem for most Photography, but when taking very close up pictures parallax (the difference between what the viewfinder "saw" and what the lens was "seeing") created a framing error. The beauty of an SLR is that you can couple it to any lens of any focal length and see what the lens sees. It can even be mounted on a telescope or make photos through a microscope, and what you see in the finder is what you will photograph.

The secret to the single lens reflex (SLR) is a moving mirror behind the lens in front of the shutter, and a pentaprism atop the housing. The light coming in through the lens is directed 90° by the mirror to the pentaprism, which diverts the image to the finder eyepiece without its being flipped, distorted or reversed. (Most SLR finders do not present quite 100% of the image, however. Typically they show at least 90% of what will be on the film, which is plenty.)

When the shutter is tripped, the mirror flips up out of the way, the finder image blacks out for the fraction of a second that it takes for the aperture to stop down and the shutter to open and record the image on film or sensor. Most modern SLR's then automatically return the mirror to viewing position.

SLR's aren't perfect. They tend to be larger and heavier than "rangefinder" type cameras, and older models can be quite noisy because of the complex action of the mirror flipping up and down (manufacturers have spent years developing quieter and quieter actions). In certain applications, mirror "bounce" can slightly degrade an image. SLR's also tend to be expensive due to their complexity, but they remain the gold standard of hand-holdable film cameras and are gaining rapidly in the digital market, especially among professionals.