Canon FD Lenses - Other Issues Part 1


Single Lens Reflex Camera Body System

There are two principle reasons why 35mm single lens reflex cameras are said to be lens-based. First, they take pictures "as you see them." This doesn't mean the subject as you see it with your naked eye, but as you perceive the image presented by the lens.

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Second, look through the viewfinder of an SLR camera and you'll see that the area you focus on looks sharp while all other areas appear blurred. Thus the modern SLR viewfinder systems have made it possible to make visual checks through the viewfinder of the actual optical changes caused by the lens in use.

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EOS SLR Illus.jpg. Here's how an SLR viewing system works. Light passing through the lens from the subject strikes a 45° inclined mirror housed in the body. The mirror reflects the image to the focusing screen. Reflected in turn by the pentaprism, the image you see by looking through the finder is correctly oriented and right side up. When you push the shutter button the mirror swings up and out of the way allowing the light to go directly to the film to form a latent image of the subject. You can see the actual image until almost the instant it is recorded on the film.

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One reason for the development of the SLR is the problem of parallax error with rangefinder-type cameras. Because the viewfinder is separate from the lens in a rangefinder camera it is possible, especially in close focusing, for there to be a difference between the lens' image and that of the viewfinder.

Known as a parallax, the result is a marked difference between the viewfinder image and the image actually recorded on the film. With SLR cameras, however, this will not happen. For all practical purposes, you will record on film what you see in the viewfinder. This feature of the SLR becomes particularly significant when the subject fills the viewfinder screen to the edges. This ability to record on film virtually all that you see in the finder is particularly important for close-up or macrophotography. With SLR cameras, extreme close-ups that would be impossible with the rangefinder system are easy. In many ways, on the other extreme, super telephotography presents another advantage where it is difficult to matched by a conventional rangefinder system.

Accurate Focusing with Any Focal Length Lens

Basically, this is because of the nature of the SLR viewing and operating concept and system, focusing is extremely accurate no matter what focal length lens is mounted on the camera. The distance from the camera's mirror to the focusing screen is the same as the distance from the mirror to the film plane.

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Focus is precise with any lens from extreme wide-angle to super telephoto or zoom lens. If you focus the image sharply on the viewing screen it will be sharp on film.

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The Focusing Screen

Let's take a look at the focusing screen and its role in the single lens reflex design. In the early days of single lens reflex development the screen was a rather simple device with a matte, or ground glass surface.

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The early screens were difficult to use, for to be effective in diffusing light their surfaces had to be rough. As SLR system has the advantage to view things 'directly', if the single lens reflex was to be a success, the viewing and focusing system needed improvement.

The design and its surface has been improved and refined over the years with new production technique employed, resulting in a considerably brighter image which greatly improved focusing ability. Additional focusing aids in the form of a microprism and split image rangefinder added to the center of the screen make the focusing system even easier to use and has a broader scope of general application for most lenses introduced later. One of the significant design was the Split image. Dividing the subject in half horizontally, the split-image rangefinder works well for subjects with well-defined outlines or vertical lines. The microprism is a big help in shooting subjects having no definite contours and can be used effectively with a variety of lenses. In addition, the matte portion of the screen is particularly useful when shooting portraits or close-ups, or when using a slower speed telephoto lens. The standard screen is designed for utmost versatility so as to cover a variety of subjects and situations. However, because the ideal screen can vary depending on the lens in use. and the subject being photographed, only selective 'higher end"Canon SLR cameras such as the Canon F-1, A-1, AE-1 PROGRAM, and the T90 in the Canon T series SLR models have such provision for interchanging focusing screens feature (out of these few, only the Canon A-1 needs the service of a technician to change its screen, while the rest were designed to be more friendly with user interchangeablility).

When the Canon revealed their flagship model,
New Canon F-1 in 1981, they have also exhibited a new manufacturing technologies that enabled the camera to have such high precision to perform the all important metering functions for the camera. The brightness level for viewfinder focusing was brought to new height despite in the case of the New F-1, part of the in coming light beam through the lens was diverted to another condenser to perform either conventional centerweighted, a 12% partial and even 3% spot metering and the viewfinder does not exhibit any loss of brightness for viewing, focusing or composing. Subsequent models have benefited from such research and know how, thus, as far as viewfinder technology relates, Canon was leading it his field.

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Illustrations above can be seen as various stages of development for focusing screens. From the early uneven, rough sandblasting surface to laser matte technology around the late '70 and early '80 and eventually improved to the all bright laser matte screen. The quick-focusing Cross Split Versatility is the concept upon which this novel focusing aid is based. The conventional split-image rangefinder works well with vertical lines, but is difficult to use when focusing subjects without them. The Cross Split solves this problem by dividing the image into quadrants for quick, easy focusing. Because it can be used to focus both horizontal and vertical patterns, the Cross Split is suitable for virtually any subject. Only Canon offers this type of focusing aid and has made SLR such a pleasure to use with all the lenses in production.

Far left indicates various possible combinations of design the spit image in a focusing screens.


The Split image enlargement.

The Bright Laser Matte screen in enlarge state is so smooth for such even light distribution as compared with older screen illustrated at the top.
A close-up of the Bright Laser Matte screen surface reveals approx. 220 million regular-shaped 22u grains that function as microscopic lenses with extremely accurate interrelation positioning over the entire surface of the screen. This honeycomb structure of micro-lenses is effective in guiding the maximum amount of light to the condenser lens.

The real benefits of these new advances are evident when shooting in low light, or when using lenses with small apertures. The overall increase in brightness makes focusing much easier and more accurate in difficult conditions. Technology as such makes makes focusing easier: there is no darkening when stopping down and they are also a great aid when shooting with slower lenses.

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Canon FD mount Camera Bodies:
A Series:
AE-1 | AT-1 | A-1 | AV-1 | AE-1 Program | AL-1
T- Series:
T50 | T60 | T70 | T80 | T90
F-1 | New F-1
Canon FL Resources
Pellix | FTQL

Lenses: Canon FL lenses | Canon FD lens resources

Canon EOS SLRs | Canon EF lens Resources


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