Today I’m sharing some disassembly and repair details on quite interesting manual focus lens in Fuji X mount – Kipon Iberit 75mm F2.4 . This lens sample is provided by Tom Brayne (thank you Tom! ) to explore how to calibrate infinity focus, which is not trivial with Iberit 75mm.
Lens focus calibration can be a trivial process if focus ring mechanics has standard design used my many modern and older manufacturers. Typically external focus ring is connected with few bolts to inner focus ring, so can be repositioned without lens disassembly. Well, Kipon Iberit is a special lens here – it has external bolts on focus ring, but they just connect textured focus ring to… second part of external focus ring with focus length scale. So traditional calibration method does not work, and soon you’ll learn what IB/E Optics designed for that.
Part one – disassembly for focus calibration
Iberit 75mm F2.4 lens has very accurate lens body design with thin lines and precise looking parts. Four bolts are holding nicely machined aluminum mount ring. I’m unscrewing them and taking mount ring off.
Then I’m unscrewing three tiny black bolts holding the DOF scale ring with large “75” number. Then removing this ring.
After removing DOF ring you’ll have access to five large black bolts on the perimeter of inner frame cylinder. All that five bolts need to be slightly loosened by making 1-2 turns of screwdriver. Then you need to attach lens mount ring and secure it back with four silver bolts.
Now attach lens to camera. Kipon Iberit 75mm F2.4 sample provided by Tom had some factory defect – it couldn’t focus to infinity, the most far focusing distance was about 30-50 meters. After loosening that five black bolts you’ll notice that focus ring now can be moved pass infinity point. So just rotate ring very slow to let it pass infinity mark and until picture in camera is focused to infinity. At this point stop, and tighten all five black bolts. Now Kipon 75mm F2.4 is able to focus to infinity, however as a side effect – the focusing scale on ring is not very accurate because it is also shifted. Well, it’s the only way to easily fix focusing precision issue. There is also more challenging way – is to grind off aluminum ring (about 1mm of inner surface metal need to be removed), but that required hours of work with metal file, or access to metal machining devices (which I don’t have). Though with Fuji X cameras it’s not critical to have precise marking on focus ring, because you are focusing through viewfinder anyway.
Last step is to remove mount ring, attach the DOF scale ring, secure it with 3 bolts, and finally attach mount ring again and secure it with 4 bolts. Done! Now you know how to relatively easy fix focusing precision issues with some Kipon Iberit 75mm F2.4 lens samples. In the next part of article I’ll show how that focus area is constructed and also explain how to deal with potential optical decentering issues.
Part two – disassembly for optical alignment
Now I’m going to disassembly Iberit 75mm further to access rear optical area. This lens sample has some optical decentering which can be observed with modified Siemens Star Chart. On the image below lens is slightly defocused on test target, to better see difference of diffused area around black circle. With perfectly optically centered lens thin line around black circle should have same thickness, but as you can see line is very thick in bottom area and very thin in top area.
In many cases rotating front optical group or rear optical group may solve the problem. NOTE – this operation can easily make things worse, so I’m performing such step only when I’m confident enough that any action I take can be undo. So in case of experimental rotation of optical module, I first mark it’s position with pencil on frame, then rotate, and then check on test stand. If it improves – good, if not – I return to original position of optics. Some lens allow that, some lens don’t. Fortunately Kipon Iberit 75mm F2.4 makes that process relatively precise and safe.
So, I’m removing mount ring, and DOF scale ring as explained in part one of this article. Next step is to focus lens to infinity, and make sure focus ring stays in that position. Then unscrew six small black bolts. They sit quite firm, so require accurate initial high torque movement by few degrees to release them, then accurately unscrew all six bolts.
Now the rear frame cylinder can be slowly extracted.
It is a solid part with two large protruding areas covered with grease – that are helicoid guiders.
It is important to insert helicoid guiders to original position when assembling lens back.
Next step is to hold focus ring with frame together and slowly unscrew focusing frame from helicoid core in CW direction. Do it slow until frame starts pull out, then mark that position with pencil.
Here’s the look to inner thread of outer focusing helicoid ring – looks like it is made of brass and machined with very high precision.
That brass cylinder is quite thick and has cut area on other side – when focusing there is a limiting pin sliding inside that horizontal opening, and limiting focus ring rotation (marked with right green arrow). Here you can also see that five black bolts securing brass cylinder to outer focusing frame, which we loosen for focus calibration and allowing to shift horizontal opening CCW or CW when aligning focusing range. One more interesting part of IB/E Optics design here – focusing ring is connected to the lens frame with bearing, allowing very smooth rotation and keep it very durable. That is smart solution, allowing to distribute lens core pressure when focusing, which is important for this front heavy lens.
Inner core of Iberit 75mm lens has long cylinder for optical anti-reflections. It need to be unscrewed CCW.
At this point you’ll have full access to rear optics for accurate cleaning if needed. Inner ring around rear glass – is a whole rear module frame, it has two grooves on edge. Second ring around that frame – is a fastening ring. If needed rear lens module can be extracted to allow access to aperture chamber for cleaning.
To perform decentering calibration experiment, first mark radial position of rear optical module. Then loosen the outer fastener ring by rotating it CCW to 10-20 degrees.
Once fastener ring is loose you will be able to rotate rear optical module. For this sample I rotated it to 10-15 degrees for first experiment.
Next step is to gently tighten the fastener ring, and partially assemble lens frame back.
Do not attach anti-reflection cylinder at this point. Align position of focusing frame with lens core and attach helicoid, then rotate focusing frame until it is back to original position.
Then insert ring with helicoid guiders and secure it with all six bolts
Finally, attach mount ring and secure it with four bolts. Now put lens on camera and test the optical alignment. In my case I noticed slight improvement.
Next step is to accurately loosen the rear optics fastening ring, and rotate rear optics module to another 5-10 degrees, then fasten ring and test lens again. After doing it few times I noticed significant improvement at some point. As you can see at test image below the line around black circrle has almost even thickness.
So at this point I’m securing the fastening ring around rear optics module, and screwing in the anti-reflection cylinder to its place, which can be done without disassembly of focusing area. Finally I’m removing mount ring, attaching DOF scale ring, then mount ring again, and now lens is assembled back and operates much better!
Conclusions
IB/E Optics engineers did excellent job designing Iberit 75mm F2.4 lens frame in the way it is relatively easy to maintain minor issues, and components are all metal and constructed to operate for very long time. I had lots of fun exploring this unique and interesting mechanical design, and glad to say that Iberit 75mm F2.4 has very good level of maintainability.
This lens sample also shows that not every lens copy actually passed quality control, so it makes sense to test your lens immediately after purchase to make sure focusing can reach infinity, and that there are no noticeable optical decentering issues. Even though lens is designed in Germany and machined with high precision in advanced Chinese factories, but to keep lens cost affordable, manufacturers still can not test and align each lens sample, and often put it at Customer Support and shipment return level, in case of defective samples.
The Iberit 75mm F2.4 is a very good lens optically and mechanically. I like its very compact size and buttery smooth focusing. Aperture ring has click stops which is another great plus. I like how sharp is this lens wide open at F2.4, but at the same time it renders OOF very smooth and balanced.
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