How Do SLR Cameras Work?
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The Basics
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Single-lens reflex cameras were a major innovation in camera design because the photographer can see the same thing the film will capture. Previous camera designs, including twin-lens reflex, rangefinder and point-and-shoot cameras, all had separate "taking" and "viewing" lenses. This meant there always would be at least a slight difference between what you saw and what was recorded on film. It also prevented the use of extreme wide-angle or telephoto lenses. The SLR fixed all this by using a single lens.
The "Magic" Pentaprism
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Pentax introduced the SLR in the late 1950s, as did Miranda. Pentax derived its name from the pentaprism that makes the SLR work. Essentially, this is a prism with five sides that takes the image and turns it right-side up and changes it from left to right so the eye-level view is the same as you would see it with your eye. Originally, before the pentaprism, the image was reversed left to right on waist-level view screens, leaving it completely upside down and backward in the earliest cameras. The pentaprism takes the image from the lens via a mirror, then corrects it and shows it to the photographer in the eyepiece.
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What You See Is What You Get
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The lens in an SLR "sees" the image and focuses it onto a mirror at a 45-degree angle. This image bounces up through a focusing screen--sometimes interchangeable with some higher-end cameras--into the pentaprism and out to your eye. When you have the image you want in focus, you push the shutter button. The mirror flips up out of the way allowing the image to pass where the mirror was. The shutter opens and exposes the film. At the end of the process, the shutter curtain behind the mirror resets with the cocking of the film wind lever and the mirror flips back down into place. With fast shutter speeds, this process is nearly instantaneous, and there is only a momentary blackout in the eyepiece.
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