CONTAX Planar T* 50mm f/1.4 Disassembly – Blackening the Lens Edge with Sumi Ink

This is a follow-up to my CONTAX Planar T* 50mm f/1.4 balsam separation repair. It is the same lens. After the two pieces of glass were cemented together again, there was one more job to do: painting the edge of the glass black. This post is a short disassembly showing that step, done with Japanese sumi ink, and the finishing touches that followed.

1. Why the edge is painted black

The edges of the glass in a lens are usually painted black. Light that hits a bare glass edge can bounce around inside the lens and show up as flare or a loss of contrast. A matt black edge soaks that light up instead.

2. Grinding the ink

For the paint I used sumi, the black ink used in Japanese calligraphy. It is made by rubbing an ink stick with a little water on an inkstone. Here are the ink stick, the inkstone with freshly ground ink, and the brush, next to the element.

3. Painting the edge

With a fine brush, I painted the ink all the way around the edge of the element. Getting the edge evenly black took some patience.

Then I dried the ink with a hair dryer.

4. Finishing touches

The next day, with the lens back together, I opened it once more to finish the job properly. From the mount side, the rear group unscrews as one unit, and the cell with the cemented element comes out of it.

To take the element out in the first place, I had lifted the crimped metal rim of its cell with a knife. Now I pressed that rim back down over the edge of the glass with a flat metal tool, a little at a time all the way around, so the element is held firmly again.

Then I touched up the inside of the cell around the glass with the same sumi ink.

Here is the element in its cell, held up to a light.

After that, the rear group went back in. As I wrote in the first post, this lens later found a new owner through our eBay shop as a Mint lens.

Thank you very much for reading.