The Carl Zeiss Jena Biotar 58mm f/2 lens is a legendary lens. It consists of 6 elements in 4 groups and it was produced from the late 1930's to the early 1960's (before it was superseded by the equally legendary Pancolar 50mm f/1.8). The Biotar is famous for its swirly bokeh. After WWII, it was cloned in Russia under the name "Helios 44-2" which is now one of the most famous vintage lenses.
There are a few different versions of the Biotar with different aperture mechanisms. Mine is the last version with a semi-automatic aperture with 10 aperture blades. When I received it, the focus was rather stiff and it had oil on the aperture blades. It turned out that it is rather easy to service.
The whole optical system (with the aperture mechanism) can be separated by opening a single ring on the backside, using a lens spanner.
Now, set the focus to infinity and remove this screw (which is setting the focus limits).
Now, slowly(!) focus closer and closer, to the minimum (of 0.6m) and beyond, until the inner helicoid becomes able to rotate freely. For my lens this happened when the "m" was at the focus marker.
For the following, I keep the focus ring at the "m" mark. As seen from the front, I rotate the inner piece (slowly!) to see when it becomes loose and remember exactly where this happens. For this, I remember the position of the "upside-down" U-shaped piece (which is at the top in the picture below).
For my lens, this piece becomes loose when the"upside-down U-shape" is in the middle between the 2m and the 2.5m marks. This is exactly where it has to be inserted later!!
Now, I document the exact position of the focus ring. For this purpose, starting at the "m", I turn the focus ring clockwise, passing the "0.6m" up to the infinity mark. Then I continue to rotate the ring clockwise, and I check how far it goes before it comes to a stop. For my lens, this is exactly one more full turn.
Now I rotate it back by one turn (so it's again at the correct infinity position) - from there I keep turning, passing the 0.6m mark, until I reach the "m" mark (where the inner helicoid separated). From here, I continue to rotate it, until it comes out (for my lens after almost 7 full additional turns).
Now I rotate it back by one turn (so it's again at the correct infinity position) - from there I keep turning, passing the 0.6m mark, until I reach the "m" mark (where the inner helicoid separated). From here, I continue to rotate it, until it comes out (for my lens after almost 7 full additional turns).
Now I got both helicoids separated - and, in the picture, already cleaned.
If you want to clean some of the glass, now is a good time to do that. You can access the rear element, by opening the ring on the back side.
To access the front glass and the aperture, you can remove the name ring with a rubber tool.
Open the outer ring (with a lens spanner) to extract the whole front glass.
Open the inner ring to access the front lens.
At this point you can also access the aperture blades. I cleaned these in-place (without further disassembly).
When this is done, the lens can be reassembled. Since you remembered the exit points of the helicoids (you did! - didn't you?) you can put everything back together in the reverse order as described before. (Of course, with a touch of helicoid grease on the helicoids!)
And here it is:
Almost as good as new. Clean on the inside and outside and freshly lubed. Ready for taking great pictures. That's why we did this!
