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Canon Rangefinders - Peter Dechert and Peter Kitchingman Peter Dechert is best known for his Canon Rangefinder, Canon SLR, and Olympus Pen books, the latter two long out-of-print. He was a monthly columnist for many years for SHUTTERBUG magazine, and has contributed to many others. Most recently he has written about the pre-WW2 Zeiss 35mm cameras, but his interests in camera equipment and optics are many and varied. As a pro protographer and honorary life member of ASMP, Peter is also expert in using the gear! Peter Kitchingman - author of Canon Rangefinder Lens book Peter Kitchingman's 'Canon M39 Rangefinder Lenses 1939-71' book is the definitive source on these very interesting optics. His interests also go to the entire Canon Rangefinder system and beyond.

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What are the different coatings on lenses?
Old 06-23-2016   #1
darinwc
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What are the different coatings on lenses?

Hi guys!
I'm new to the Canon rangefinder world. I have a Canon sales booklet that lists the different lenses available at the time. They are noted as having amber, magenta, or purple coatings.
Does anyone know what the difference is?
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Old 06-24-2016   #2
Dwig
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Thickness ...

Optical anti-reflection coatings were still a bit of a challenge in the early '50s, but by the end of the decade they had evolved significantly. The thickness of the layer determines the dominant color seen in reflections. The thickness is determined by the exact chemical makeup of the coating. As coating tech evolved, new coating varied in thickness. The color you see is not a tinting, it is the result of interference between the light reflecting from the two surfaces. Google "Newton's Rings" for the explanation of the physics involved there.

The magic of such coatings is that while a single-layer coated lens has twice as many reflections, one from the air-coating boundary and one from the coating-glass boundary, the total brightness of those two reflections is less that of a single air-glass boundary. This happens when the coating has a refractive index between that of air and glass.

Someone, I believe it was a Schneider-Kreutznach, figured out that the best results occurred if the refractive index of the coating was exactly half way between air and the particular glass being coated. Since part of the magic of optical lens systems is that they use a mix of glasses of differing refractive index, coatings aren't a one-size-fits-all thing. They developed a variety of coatings, each one optimized for a different glass. This idea quickly spread throughout the better optical manufacturers.
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Old 06-24-2016   #3
darinwc
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Wow! Thank you for the detailed reply!
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