 BetterPhoto Member |
SLR What exactly is the difference between SLR and 35mm?
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doug Nelson |
Michelle, SLR stands for single lens reflex, a 35-mm camera design that allows you to look right through the lens when you compose your picture and focus. This design also makes interchangeable lenses easier to design and manufacture. Most consumer cameras these days are 35 mm, but not SLR's. They have their viewfinder window above and to one side of the lens. The old term for this design is rangefinder, because you formerly had to focus using this little window.
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Jeff S. Kennedy |
One minor correction to what Doug said. Most of the current point and shoot cameras do have viewfinders above and/or to one side of the lens but they are not necessarily rangefinders. Rangefinder is not the old term for this type of camera. A rangefinder is a design where the lens is coupled to the viewfinder and by means of a "ghosted" image you are able to focus. Many of today's p&s cameras are autofocus and have nothing to do with rangefinders.
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Kris Haskins |
I've often wondered how the rangefinder cameras couple the lens to the viewfinder. Can anyone explain how they do it?
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John A. Lind |
I believe you are familiar with the triangulation method to measure distance. If you know the length of a right triangle base and the angle between the hypotenuse and the base, you can find the length of the other leg. A rangefinder has two windows on the front, one for the viewfinder and the other (typically smaller with a green or yellow filter), separated by several inches (on a camera). This is called the "base." The longer the rangefinder base, the more accurate the distance measurements. The second window provides the overlap image as a spot in the center of the viewfinder and gives a view along the triangle hypotenuse. Its image is reflected over to the viewfinder via a rotating mirror or prism. The viewfinder has a fixed, partially silvered mirror or another prism to superimpose the two images. For those RF's bodies with interchangeable lenses, the focus helical is typically on the camera body, not the lens. As you focus the lens, the rotating mirror (prism) rotates changing the view provided for it. Most have a "focusing wheel" and all have a precision gear train between the rotating mirror (prism) and the lens focusing helical. When the focus wheel is turned, both the lens helical and rotating RF mirror turn simultaneously. Obviously, the rotating RF mirror (prism) and focus helical alignment is critical. Usually they are set at infinity. The helical is adjusted for infinity position by mounting the camera without its back and using a piece of frosted glass over the film gate (frosted part facing forward toward the lens). The camera is aimed at an extremely distant object and the helical adjusted for critical infinity focus on the ground glass viewed using a high power loupe (much as you would do on a view camera ground glass). Then the overlapping image in the RF is adjusted so that it aligns properly. I've uploaded a small diagram showing how the triangulation works between the the viewfinder window and rangefinder windows. -- John
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