Single-lens reflex features
In a single-lens reflex camera the viewfinder image is formed by the camera lens. Thus, the viewfinder image is always the same, allowing for framing tolerances, as that which subsequently appears on the negative or slide. No separate viewfinder is necessary when the camera lens is changed or when attachments such as close-up lenses, extension tubes, bellows or tele-extenders are used.
In the older single-tens reflexes, this principle was simply achieved by placing a movable mirror at a 45 degree angle behind the lens to redirect the image through 90 degrees to a ground glass screen in the top of the camera. The mirror was so placed that the distance from its surface to the film |
and to the viewfinder screen were the same. Thus, the viewfinder screen could also be used for focusing.When the image was accurately focused on the screen it must also be in focus on the film.The 35 mm camera for example is nearly always used at eye level so an ingenious five-sided prism (pentaprism) was designed to sit on top of the focusing screen to turn the image-forming rays through another 90 degrees so that the user could look in the direction of the subject while he took his picture. But that is not all the pentaprism does. It also corrects the lateral reversal that would be present if simple mirrors were used, so that the viewer sees an upright, right-way-round image on his screen.
The final part of the 35 mm single-lens reflex viewing system is the eyepiece lens, focused on the |
viewfinder screen via the reflecting surfaces of the prism. This is a magnifying eyepiece which enables the viewer to see an image that appears nearly life size when the standard 50 mm lens is attached to the camera.
Single-lens reflex operation
The main refinements that have been added to the original single-lens reflex design are the instant return mirror and the automatic diaphragm.Early cameras of this type were rather slow in operation and a little disconcerting to the user. Although the mirror automatically flipped up out of the light path when the shutter release was pressed, it stayed there, blacking out the view through the eyepiece until the film was wound on for the next exposure - an operation that also tensioned the shutter. |
Similarly, as it is always advisable to view and focus the image at its brightest, the user had constantly to open and close the lens diaphragm between shots. Inevitably, he frequently forgot to stop down after
focusing and overexposed his film by shooting unintentionally at full aperture.The modern single lens reflex overcomes both of these problems. The mirror not only rises automatically just before the shutter opens but also returns to the viewing position when the shutter closes. The viewfinder blackout is barely noticeable at the faster shutter speeds.
The diaphragm setting problem was overcome by linking the operation with a simple push rod or sliding lever mechanism emerging. |
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