CONTAX Carl Zeiss Planar T* 50mm f/1.7 (MMJ) – Re-cementing a Doublet with Balsam Separation

This CONTAX Carl Zeiss Planar T* 50mm f/1.7 is the later MMJ version for the C/Y mount. It came to me with balsam separation, and I repaired it in July 2024.

Some lens elements are made of two pieces of glass glued together with a clear optical cement, often called balsam. With age the cement can start to let go. When it does, you may see haze, a cloudy edge, or patterns inside the glass, and pictures can lose some contrast. Cleaning the outside surfaces will not help, because the problem is between the two glasses. The fix is to take the pair apart, clean off the old cement and glue them together again.

Here is how the job went, in the order the photos were taken.

1. A lens group out of the lens

First, a lens group came out of the barrel. Here it is from both sides.

One element lifts out of the front of the group. Holding the group up to the light shows the balsam separation: a fine, frosty haze across the glass.

2. Retaining ring and the next element

Inside the group, the next element is held by a thin retaining ring. With the ring off, the element comes out of the cell.

3. Freeing the cemented pair

The cemented pair was held in its cell by a crimped rim. Wearing gloves, I used the blade of a utility knife to carefully lift the crimp, and the pair came out. In the second photo you can see the thin line on its edge where the two pieces of glass meet.

To keep things in order, the parts went into small bags labeled in Japanese, “rear 1” and “rear 2”.

4. Separating the two elements

To separate them I used heat: I wrapped the pair in cling film and simmered it in a pot of water. The heat loosened the old balsam, and the two elements came apart. One of them has a blackened edge.

5. Cleaning

The old balsam has to come off completely before new balsam goes on. I cleaned it off both elements with anhydrous ethanol, and handled the glass with gloves from here on.

6. Re-cementing with balsam

For the new cement I used xylol balsam. I laid one element down, put one or two drops of balsam on it, and set the other element on top. Then I pressed down on it to push out any air.

To keep the two elements centred on each other, I set the pair between the jaws of a caliper so their edges lined up. Wrapped in tissue, the pair was held in a spring clamp (a fairly strong one) for about a week, to give the balsam plenty of time to cure.

7. Blackening the edge

After cementing, I painted the edge of the element black. This is called edge blackening.

With the separation gone and the edge blackened, the glass is now very clear.

Finally, the lens went back together in reverse order.

Balsam separation looks serious, but on many lenses it can be repaired by re-cementing the pair.
Thank you very much for reading.