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Nikon RF - Contax RF Lens Compatibility

IF the Nikon I used the Zeiss Contax mount, why isn't it fully compatible with Zeiss Contax Lenses?

If it quacks, it might NOT be a duck. Much to the irritation of later photographers, the lens mount on Nikon rangefinders LOOKS like a Zeiss Contax mount and MOUNTS  Zeiss Contax lenses, but is NOT fully compatible with Zeiss lenses.    Sometimes the pics can be out of focus.  Hmm.  Both the internal mount and external mount lenses fit just fine.  The lens  back focus is the same. How could that happen? 

Well, like many questions, the answer depends  upon how you interpret the data.  For me, the fly in Nikon's soup is Canon -- a situation that continues to this day!   Nikon had never built a 35mm camera before, but  before the war and up to early 1948 supplied 50/3.5 or 50/4.5 Nikkors for the Canon Hansa, S, J, NS, JS, S-1, J-II, S-11.  The Canon is a Leica copy, using 50mm lenses of a nominal focal length of 51.8mm.  The Zeiss Contax uses a nominal focal length of 50.0mm.   In one of its biggest foul-ups ever, Nippon Kogaku decided to use the 50/3.5 Nikkor already developed for the Canon, for the Nikon.   Remember there was no guarantee in 1948 the new Nikon camera would be a success.  Perhaps due to lack of space, or investment capital, the decision was made to use the Leica by way of Canon inspired 51.8mm as the Nikon I's nominal focal length. If the new camera  went belly up, Nippon Kogaku  could  continue to market Leica screw mount lenses as a backup -- which is exactly what Nippon Kogaku did until the late 1950's.  

OK, the differences between 51.8mm and 50mm, so what?  Well, remember that the Zeiss Contax uses a focusing helical IN THE CAMERA body for normal lenses, so the Nikon I used the same type of helical. Alas, what happens when you put a 51.8mm lens in a focusing mount intended for a 50.0mm lens?  While infinity is in focus, the other distances are slightly off focus, especially at full aperture and close up -- where the difference can not be carried by depth of field.   More than that, the amount of helical travel for a 50.0mm lens will not focus a 51.8mm lens to the same standard 3.5 foot close focus distance. 

The solution? Just increase the travel of the Nikon helical so the 51.8mm lens can be focused to 3.5 feet, and adjust the rangefinder mechanism accordingly.  

The problem created by the "solution"  is that ALL Nikon lenses will interact with the longer throw helical in the Nikon body, that was originally modified for the Canon 50/3.5 Nikkor lens.  For 35 and shorter lenses, the difference is so small as to normally be considered inconsequential, allowing for the interchangeability of Nikon and Contax wide angles.  With 50mm and longer lenses, the difference begins to add up so that wide open and close up, Nikon and Contax lenses are not compatible. But if you shoot at medium and longer distances, or at medium or smaller apertures, depth of field will usually take care of the focus error.  As focal length increases, the problem increases, to the point that Nikon made special 85/2, 105/2.5, and 135/3.5 lenses for Contax, so marked on the lens barrel with a "C."

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Revised: November 26, 2003 Copyright  2002  Stephen Gandy. All rights reserved.    This means you may NOT copy and re-use the text or the pictures in ANY other internet or printed publication of ANY kind.  Information in this document is subject to change without notice.  Other products and companies referred to herein are trademarks or registered trademarks of their respective companies or mark holders.