Adding Corrective Optics to an Film-Era Lens
Adding a plano-convex lens to a Zeiss Biogon to correct field flatness to modern standards.
In Part One, I described how to deal with vignetting and lens cast in some older lens designs when they are used on digital camera.
That leaves two other steps, which involve modifying the Biogon lens:
Modify the position of the Zeiss Biogon lens core so the new assembly can reach infinity focus.
Add a plano-convex (PCX) lens in front of the camera lens to correct for field curvature.
WARNING: You are partially disassembling an expensive lens. Never proceed until you completely understand how to safely perform each step!!!
The images above are in sequence, from 1 to 9:
A type of spanner wrench for removing the lens layers. This one is from Newport.
Another type of spanner, commonly found inexpensively on Amazon.
The slots on the back end of the lens where the spanner points will go. This is a collar that locks the lens core into the base of the lens assembly.
Inserting the spanner points into the locking collar. Note: good idea to put something soft under the face of the lens just in case. The first layer of glass is recessed, but you never know what can happen. Non-slip materials are ideal.
After the collar is loosened with the spanner wrench, you can remove it by hand. (Turn by hand once it’s loose, then set it aside in a safe area.)
With the entire lens sitting on a flat surface, lift the lens base off of the core. Take GREAT CARE with this step. You don’t want to drop or damage anything. There may be some initial friction when you attempt to remove the core; if so, use one hand to bold the core near the front of the lens, and the other to remove the base. Go slow, figure it out as you lift, feel for any gotchas as you slowly lift up the base. STOP if you have the slightest hint of doubt. This is a dangerous step; you should study the lens until you are sure you understand what to do.
The exposed lens core. The shims are on the shoulder in the middle of the photo. These are what you need to remove to allow the lens to reach infinity focus with a PCX lens attached. NOTE: the shims may vary in number and thickness; they are selected by a technician working on an optical bench to make sure the infinity stop is in the right place. See note below; you may or may not get infinity focus by simply removing all the shims!
Lift off the shims VERY CAREFULLY. They are super thin, and are easily damaged. Take your time, and when you have them off, be careful not to squeeze or bend them. Place on a flat surface; later, put them in a safe place, such as a plastic box with foam, a sturdy envelope that will be put in a safe place, etc.
The lens core on the left; the shims on the right.
NOTE: in my case, removing all of the shims exactly works to provide focus with the PCX lens I used (a 50.8mm one, with a focal length of 1000mm). It’s possible you will need to use a different PCX lens focal length or diameter for your build. That may mean either only removing some of the shims, or finding a source for new shims of the correct thickness and inner/outer diameters!
NOTE: other typical focal lengths that have worked on other lenses are 1200mm, 1300mm, and 1500mm. Its possible that one of these focal lengths would work better than the 1000mm I used, but none of those were available from Thorlabs so guess what: I went with what I could get, and it works to my satisfaction.
Everyone recommends using a PCX lens from OPtoSigma, but the shipping costs were ridiculous, over $50, nothing cheaper. So I ordered mine from Thorlabs, it’s at this link. That version is unmounted (bare); they also make a mounted version in their custom SM2 thread. But adding an adapter to get to the SM2 thread would put the PCX lens too far in front of the Biogon lens…so I did not use that. This complicated mounting—I document the mounting method I used, but it’s not ideal, and I will update this post if I find a better method.
If you ever want to put the shims back in, you follow these same steps. Note that there is likely some grease on various surfaces; don’t touch anything that is internal other than lifting the shims off. Use cotton gloves if you are unsure what you can and cannot touch. Don’t be crazy like me and use bare fingers. I have worked for several decades in optics, but that doesn’t mean I do everything by the book - you should do it by the book, or don’t do it at all. I don’t want any late night emails with horror stories; make sure you are clear about each step and only execute the step if you know what to do.
Note there is a T slot in the base of the lens. There is a pin on the core that must go into this slot.
The pin on the core.
Placing the base onto the lens core.
Replace the collar—screw it on with your fingers, then tighten with the spanner wrench.
There are multiple ways to build a holder for the PCX lens. The Biogon 21mm lens has a 46mm filter thread. I purchased a 46-52mm adapter; the PCX lens has an outer diameter of 50.8mm, so I figured (wrongly) that a 52mm thread would leave room to put an a ring in to lock the lens in place. So I used bathroom silicone adhesive to attach the glass to the adapter. (Not least because I should be able to force this apart and rebuild it when I figure out how to use a retaining ring…)
I bought an AMOPOFO 46M—>52F adapter.
What the adapter looks like with the glass glued in place , 46mm side up.
Adapter with the 52mm side up; you can see my (slightly sloppy) glue spots.
Note that the glass is thicker than the adapter. Wah!
The assembly attached to the filter thread of the lens.
I used a 54mm push-on cap to keep the dust off the PCX lens when not in use.
I added an M to Sony adapter to the lens to use it on my α1.
The images below show the before and after of the PCX lens; the corners are now sharp; field curvature has been corrected.