Disassembly and focus shift – Carl ZEISS C Sonnar T* 50mm F1.5 ZM lens

This disassembly guide shows how to access aperture chamber of Carl Zeiss C Sonnar T* 50mm F1.5 ZM lens. One of my sample had very large (about 2mm long) dust particle, so I opened it for cleanup. I’ll also display rear disassembly for focus shift optimization.

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Front area disassembly

First step is to unscrew the front black ring with “Carl Zeiss” words. I’m using rubber cylinder for that. Typically there is little glue holding the ring on place, so you may need high torque initial effort.

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Three black bolts are holding silver lens hood ring, unscrew them and remove the ring.

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Next ring around front lens is also secured with tree bolts. Once you remove it as well Zeiss unveils more interesting area. There are three brass plates that have double function – securing the front lens groups frame, and aperture ring brass lock circle.

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Put lens facing front glass up, and accurately unscrew that three bolts, they are also secured with glue, so initial high torque effort is required.

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Now the important part. Carl Zeiss optics has very high quality of glass surface geometry, it means if you rotate the glass the image quality will not degrade. But just in case I’m marking lens case position with red sign to put it later back to the same position.

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Gently remove the glass frame, now you have access to the aperture and inner glass surfaces.

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I’m typically using Rocket Blower to remove all large dust particles.

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Now open the aperture to also clean another lens surface if needed.

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Then assemble the lens back in reverse steps order.

Focus Shift optimization

Zeiss Sonnar T* 50mm F1.5 ZM lens has significant focus shift (changing aperture value shifts focus). Typically Zeiss factory optimize lens for precise focus with Leica M rangefinders at F1.5 or F2.8 apertures. It’s quite easy to disassemble rear area by unscrewing single ring with spanning wrench (hold the front lens area during that process, not the helicoid area). Then you can simply remove one of three calibration shims to set lens from F1.5 to F2.8 optimized mode. If you only have 2 shims, then it’s possible to just make extra one from aluminum metal can of same thickness wall (use caliper for precise measurements). I found information about these shims on related Flickr discussion.

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