Canon FD Lens, Manual Book

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In order for a lens to operate smoothly and harmoniously with a
camera, the coupling mechanism must be precisely built.
Lenses produced by camera manufacturers are designed with
signal pins, mounting mechanisms, and focus detection
mechanisms which ensure that the cameras will always live up
to their full potential. After-the-fact lens manufacturers,
however, do not always consider factors such as focusing
screen optics, and often the image will appear to be in focus in
the center of the viewfinder, but out of focus around the edges.
This kind of problem typically occurs because the lens is
designed without due consideration of the characteristics of the
camera body.
Performance check 4: Canon FD lenses, designed to the exacting standards of
the F-1
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Accurate focus is achieved with
FD lenses and Canon SLRs. With lower-quality lenses,
the image is out of focus.
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