Lens Coatings, Design Variants
The earliest breech-lock Canon FD lenses (1971-1973) are generally recognizable by a chrome (silver) filter ring at the front (this ring includes its own bayonet used to mount an appropriate lens hood). Nicknamed 'chrome nose' lenses, these used two new proprietary lens coatings, designated "S.C." (Spectra Coating) and "S.S.C." (Super Spectra Coating). These were both multi-coatings, but indicated two quality grades. In the 'chrome nose' series, only the large-aperture 55mm f/1.2, 50mm f/1.4 (regular and 'AL'-type) and 7.5mm Fish-Eye lenses used S.S.C. coating. On these lenses the breech ring can be rotated freely without mounting it on a camera body.
The second series of breechlock FD lenses (1973-1979) lenses is inscribed "S.C." in white or "S.S.C." in red on the front of the lens mount. The S.S.C. coating was extended to most lenses in this series. The basic S.C. coating was, for the most part, limited to the more inexpensive lenses. According to Canon literature, the selection of S.C. or S.S.C. coating was made based on the lens's requirements for flare control, etc. There are two variants of this second series. In the earlier variant, the Auto-Exposure indicator was a green circle, as it was on the chrome nose lenses. In addition, the breech ring could be rotated without mounting the lens on a camera. The later variant used a green letter "A" to indicate that the lens was set for AE operation, and on these lenses the breech ring is latched in the mounting position until pressed to the camera. Additionally, a spring causes the breech ring to rotate a few degrees upon mounting, making it somewhat easier to mount the lens by having it partially secured as soon as it is pressed onto the camera body.
In 1978, with the introduction of the New FD series (nicknamed 'FDn'), the coating type was no longer specified on the lens front. All of these lenses received S.S.C. coating, with the sole exception of the 50mm f/1.8 lens.
The original breech-ring FD lenses and the New FD lenses are completely functionally interchangeable, and may each be used on any FD camera body.
Read more about this topic: Canon FD Lens Mount
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