What Is a SLR Camera?
An SLR or single-lens reflex camera is a camera that uses the same lens to view the image and take the photograph. This is accomplished by the use of a mirror set at a 45-degree angle behind the lens that flips up when the shutter button is pressed. The SLR has a long history and has transitioned from film to become the standard in advanced amateur and professional photography.
History
Technically, the first cameras, known now as view cameras, were single-lens devices. The photographer looked at a piece of ground glass on which the lens focused the upside down and reversed right to left image. Yet the first true SLR had its patent applied for by Thomas Sutton in Britain in 1861. Still, it wouldn't be until the middle of the 20th century that the SLR camera became reliable enough for professionals. By the 1930s, some German manufacturers, including Exakta, were producing smaller, hand-held SLRs and Noviflex started making medium-format (negative size 2.25-by-2.25 inches) SLRs. Innovations in the 1950s, including a workable pentaprism to correct the image left to right and right-side up by Pentax and the introduction of "system" cameras by Nikon, led to the SLR becoming dominant in photography. By the 1970s and 1980s, the SLR also was the standard for advanced amateurs.
Function
Essentially, an SLR has the same lens for viewing the image and taking the photo. The lens, which usually has an aperture ring to determine the amount of light being let in and a focusing ring, is attached to the front of the camera. The image is projected to a 45-degree mirror on springs between the lens and the film plane. The mirror reflects the image upward through a ground-glass screen to a pentaprism or waist-level viewfinder. The pentaprism bounces the image inside it, then presents a corrected image to the photographer through the viewing lens at the back. When you turn the focusing ring, the image comes into focus on the ground-glass screen. When you press the shutter button, the mirror flips up out of the way and the shutter curtain, which is behind the mirror opens and closes according to the shutter speed set. The mirror then flips back down to view the next image.
Medium-Format
There are two forms of SLRs in use now. The choice of professionals for high-end work, such as for magazines and commercial use, is the medium-format camera, such as the Hasselblad, the Bronica or the Pentax 645. The Hasselblad, for example, is a truly modular design. The camera itself is a square with a lens mount at the front, a ground-glass screen at the top, a wind knob on the right and a shutter curtain at the rear. To this, you can add various lenses from wide-angle to telephoto, a waist-level or pentaprism viewfinder and a variety of film backs. These film backs can take 12, 24 or even 70 standard film negatives measuring 2.25-by-2.25 inches. The 70-shot film back was used by the astronauts during the Apollo moon landings. There also are other film backs to change the shape of the negative and get 16 shots or to use Polaroid film to get an instant preview of the image. The film backs can be changed in mid-roll. The lenses have leaf-style shutters within them, which sync with the camera shutter and with a flash unit at all shutter speeds.
35mm
The second kind of SLR, and far more commonly in use, is the 35mm-style camera. These work the same way as medium-format cameras except the viewfinder usually isn't removable and most of them don't have different film backs. The advantages over medium format, however, made them the industry standard. They are less expensive, much lighter in weight, have far more lens possibilities and shoot photos much faster. They hold 12-, 24- or 36-exposure film rolls.
Digital Transition
Some of the most popular SLR camera producers managed to make the transition to digital. Hasselblad first allowed the use of digital film backs to be connected to the traditional film camera, then developed very high-end all digital cameras with some of the best image resolution available, nearly rivaling film. Nikon, Pentax, Canon and Olympus all made the transition to digital with their 35mm-style cameras. Digital SLRs work essentially the same way as film SLRs except there is a charge-coupled device (CCD) or other digital capture sensor that takes the image. This vastly increased the number of images you can shoot by adding memory cards. Most digital SLRs also magnify the image, so a 50mm "normal" lens on many of them becomes a 75mm portrait lens.
Tags: film backs, ground-glass screen, 25-by-2 inches, focusing ring, same lens, shutter button, shutter curtain