Biometar 120 – Stuck Aperture Repair

(A factual, no-nonsense walkthrough)

The Starting Point (2013 Recommended Method)

An old 2013 forum post described the standard “minimal-risk” rear-access cleaning:

1. Unscrew 4 screws near the mount and lift off the mount.
2. Remove spacer ring (black side pointing up).
3. Unscrew rear lens block by hand (or spanner).
4. Remove 3 screws located on the inside.
5. Lift off the rear lens barrel.
6. Use Isopropanol (99%) to dilute dried lubricant on the iris blades.
7. Actuate the aperture lever repeatedly until the aperture ring works again.
8. Reassemble in reverse order.

Important:
The small springs under the aperture coupling fork are fragile — the warning in the 2013 post is correct.

✔️ What is correct in that method

• The 4 screws around the mount are the right entry point.
• After mount + spacer removal, the rear optical block unscrews as one sealed unit.
• Under it sits the inner barrel with three screws — removing it exposes the aperture.
• From there, a “flush clean” with IPA is possible.
• The method avoids touching the front optical block or any collimation-critical parts.
• The springs are tiny; caution is absolutely valid.

✔️ Why the rear access works (in many cases)

On the Biometar 120/2.8, the iris sits:

• directly behind the front optical block
• but is fully reachable from the rear once the rear block is removed
• with no helicoids, front-cell adjustments, or shim stacks exposed

This makes rear-cleaning the standard safe approach for beginners — and it fixes about 90% of sticky Biometars.

Initial inspection: visible oil on blades and uneven sheen indicating contamination before cleaning.

Rescuing My CZJ Biometar 120mm — A Honest, Technical, Stressful Repair Story

Introduction: A Stuck Aperture after Desert Hike

In February 2025, I bought a Carl Zeiss Jena Biometar 120mm f/2.8 advertised as “CLA serviced.”
It looked fantastic — original leather case, clean cosmetics, and no visible optical defects on the negatives.

Under a torch light, I noticed some cleaning marks and a small lint particle inside the lens.
The seller reassured me this was probably from transport, said the lens had been tested, and early results confirmed this: perfect negatives, sharp images.

Months later, after a desert hike in Arizona, the aperture suddenly stopped working.
I didn’t notice during the hike because the aperture ring still moved — but most negatives came back heavily overexposed.

During normal exterior cleaning, I realized the truth:
the aperture was completely stuck wide open at f/2.8.

Why this wasn’t surprising

CZJ lenses are notorious for stuck apertures:

• Biometar
• Flektogon
• Sonnar
• Pancolar
• Tessar

It’s an extremely common issue. But, I simply didn’t know.

Why this happens

Heat + old grease + sloppy “CLAs” = lubricant migration.

Desert temperatures likely caused old or over-applied grease to migrate into the iris.
When I opened the lens later, this theory was confirmed.

What this repair story covers

• the symptoms
• the “why” behind each step
• what turned out to be normal
• what indicated real failure
• moments of panic
• contradictions in online advice
• the rear-only cleaning attempt
• the front-side cleaning that became necessary
• the graphite application
• and why the lens now works better than when I bought it

Phase 1 — Research Chaos and Contradicting Advice

Why this phase matters

Before touching the lens, I had to understand typical CZJ iris failures.
These lenses are known for:

1. Oil migration from helicoid grease (accelerated by heat)
2. Over-lubrication during amateur CLAs
3. Oil creeping into pivot slots
4. Dried grease varnish binding the iris ring

This problem is so common that many Pentacon Six owners consider it a “rite of passage.”

The conflicting advice I found

• “Just clean from the rear.”
• “Never open the front.”
• “Always open the front — rear cleaning is insufficient.”
• “IPA damages blades!” (false).
• “Flood with IPA, works great!” (half true).
• Czech/Slovak veterans recommending ether.
• Others recommending full iris disassembly.

✔️ The safe first attempt

(And the one that should fix most lenses)

• Rear side upward
• Open the iris with the lever
• Apply tiny IPA drops at blade edges
• Cycle the lever repeatedly
• Let dry fully
• Reapply only minimal amounts if needed

This works in about 90% of stuck Biometars.

✔️ If it still sticks

A known issue:
The coupling lever plate under the mount may need a quarter turn of loosening.
This is common on older Biometars with worn tolerances.

Conclusion from Phase 1

There is no universal CZJ fix.
Every lens requires actual diagnosis, not assumptions. —

Phase 2 — Rear Disassembly: First Access to the Iris

Why start from the rear?

Because rear access:

• is non-destructive
• exposes 60–70% of the iris
• is the correct first step for light contamination
• avoids disturbing the optical groups

This method also matches old repair threads for CZJ and Meyer lenses.

After removing:

1. Pentacon Six bayonet
2. Spacer (my lens didn’t have one, so it seems depending on the model, production year or whatsoever)
3. Rear optical block

…I finally saw the iris.

What I saw (and what it meant)

Two immediate red flags:

1. Oily, uneven sheen on the blades
   • Normal: subtle silver/blue/gray
   • Not normal: wet reflections, streaks, darker patches
   • Diagnosis: oil contamination
2. Oil residue around the lever spring
   • Classic sign of over-lubrication during a previous “CLA”

At this moment it was obvious:
Lubricant had migrated into the iris — almost certainly from heat in Arizona.

Initial inspection after Phase 1: rear access shows uneven blade coloration, oil patches, and early sticking behavior before the aperture fully locked.

Phase 3 — First Cleaning Attempt (Rear Only)

Why this often fails with heavily oiled CZJ lenses

Rear-only cleaning reaches:

• rear blade surfaces
• some overlap edges
• partial pivot slot access

But it does not reach:

• blade fronts
• pivot slots completely
• ring channels
• blade-to-blade underlaps

I began with micro-drops of IPA using a toothpick. One of the methods mentioned online.
Later I used pointed cleanroom foam swabs — far superior. You can easliy control the amount of IPA and control where you drop.

What happened next

After the first cleaning:

• blades opened via ring
• closed via lever
• action felt “snappy”

And then:

• stiff
• sticky
• locked
• lever required force
• blades bowed a bit when moving
• eventually no movement

That was the first real panic moment: “I broke the iris.”

But this was just IPA doing its job: IPA dissolves oil → redistributes it → evaporates → residue becomes gummy.

This isn’t failure — it’s the midpoint.

Phase 3: Rear-only cleaning attempt. The iris became briefly snappy, then sticky again as dissolved lubricant migrated into the pivot slots, confirming that rear-access cleaning was insufficient.

I made another attempt to clean and applied more IPA and was able to move with the lever. I coud open with the aperture ring but not close. That was a clear sign more of the oil was still sitting somewhere. The need to open the front was obcious and I ordered the tools. Did I panic? For sure. But hey, I don’t have the skills (yet) but the right tools and 2 great youtube videos showing a complete teardown (which I still try to avoid).

Phase 4 — Committing to a Proper Fix: Opening the Front

Why front access becomes necessary

If the blades and pivot slots and guiding tracks are flooded with oil from a to generous CLA, there is only one chance, open the front. This sounds scary, but as you will see, it’s less scary then one think.

The Biometar 120/2.8 aperture sits between the front and rear optical groups, and serious contamination often reaches:

• the front side of the blades
• the pivot slots
• the blade guide tracks
• the inner iris ring channel
• the overlap points that cannot be reached from the rear

These areas are completely unreachable from the rear, so the only option was to open the lens from the front.

How the CZJ Biometar 120/2.8 front assembly is built

The Biometar’s front is mechanically simple:

• The name ring stays in place — it does not secure the optics.
• The front optical block itself threads into the barrel and acts as the only retaining assembly.

There are no intermediate rings, collars, or retaining hardware.
The entire front group unscrews as one sealed unit.
As long as that sealed block is not opened, infinity focus cannot be affected.

The actual, correct disassembly order

1. Remove the CZJ name ring with a rubber lens remover ring. Typically this works very easy, light pressure and the ring should move. Remove the name ring.
2. Identify the two opposing slots machined into the front optical block.
   These slots are the engagement points for the spanner wrench. Be careful to select the right tips for the spanner!
3. Insert the spanner tips into the slots and apply gentle torque.
   Once the block “breaks free,” it rotates smoothly.
4. Unscrew the entire front optical block.
   The block has long, fine threads, so this takes several full turns.
5. Lift out the complete front group.
   Nothing else holds it in place.

After removing the front group, the entire aperture assembly is exposed from the front.

What I saw

The source of the sticking became immediately obvious:

• oily streaks on the front blade surfaces
• residue inside the blade channels
• contamination in the pivot slots
• darkened overlap areas
• thickened lubricant in the inner iris ring track

This confirmed what the rear-only cleaning had already hinted at:
the contamination was present on both sides of the blades, and across the entire aperture mechanism.

Rear-only cleaning could never have resolved this.

Phase 4: The front optical block removed as a single unit, exposing the iris from the front and revealing the full extent of contamination on blade fronts, channels, and pivot areas.

Phase 5 — The Real Cleaning (Front + Rear)

Why this method works

Once the front optical block was removed, it became clear that most of the contamination was sitting where rear access could never reach:

• front blade surfaces
• inner blade overlap points
• pivot slots
• the iris operating ring channel
• the surrounding inner barrel surfaces
• old graphite and grease residue inside the front chamber
• fine particles sitting on the internal walls

Cleaning from both sides was the only correct approach.

The structure of the Biometar optical groups

Both the front and rear optical groups are sealed cells.
They are not designed to be opened without proper tools and optical bench alignment.

Opening a sealed group risks:

• permanent decentering
• trapped dust between elements
• loss of factory collimation
• misalignment that cannot be corrected without specialized collimators

Because I never opened either optical group, infinity focus remained perfect.

Cleaning the barrel and front chamber

Inside the front chamber (the space between the optical block and the iris), I found:

• graphite dust
• old lubricant residue
• loose particles
• oil mist that had migrated forward

This is common with CZJ lenses.
Many East German lenses used graphite as a dry lubricant on the iris rails and rings — over decades it settles in the threads and the front chamber.

To clean it:

• I used IPA-dampened pointed foam swabs to wipe graphite-contaminated barrel threads
• I removed the loose residue inside the front chamber
• I made sure no graphite chunks, dust, or fibers remained
• I kept all IPA applications localized to avoid migration into the optical elements

Cleaning the iris from both sides

Using:

• 99% IPA
• pointed cleanroom foam swabs
• toothpicks for pinpoint application
• flashlight

I cleaned:

• the blade fronts
• the blade backs
• all overlap edges
• each pivot slot
• the pivot ring channel

Important: everything was done by tapping, not wiping sideways.

Sideways wiping risks:

• bending blades
• catching blade edges
• destroying the overlap geometry
• scratching the metal

Capillary action did the work — not force.

Cleaning the front and rear lens surfaces

With the optical groups still sealed:

• I cleaned the rear-side of the front group
• I cleaned the front-side of the rear group

Using:

• Zeiss lens wipes (safe, no detergents)
• very light pressure
• no contact with retaining lips or shims

The front chamber contaminants were wiped away without lifting the lens elements.

Why graphite returned during cleaning

When I wiped the inner barrel and front threads, the swab turned black.

This is normal.

Biometar and Meyer lenses frequently used a mix of:

• dry graphite powder
• graphite-infused grease
• thin machine oil
• and, in some cases, factory-applied dry lubricant on the iris rails

Decades of operation redistribute this material.
IPA dissolves and lifts it.

After cleaning

After several cycles:

• the lever movement became smooth
• the blades finally closed reliably
• the aperture snapped instead of hesitating
• resistance disappeared
• the ring tracked the iris correctly

This was the point where I knew the aperture could be fully restored.

Next step was to let the lens sit for a while so all IPA could evaporate completely — basically, to make sure everything was truly dry.

After this, the reassembly began.
Was I nervous? Yes, of course. At a certain age it’s not exactly easy to handle these tiny screws.

I started with the rear section, reassembling everything in reverse order. Then I checked the small pin that engages the Pentacon Six TL mechanism to pull the aperture lever.

And then came the next panic moment:
the pin detached from the pushing lever.

This part is absolutely critical — and very easy to overlook.
The small springs had slipped over the pin plate where they are supposed to sit. Getting them back into the correct position took time, patience, and a fine tweezer.

Once that was sorted out, the rest of the assembly went smoothly, and the Carl Zeiss Jena Biometar 120mm f/2.8 finally came back to life.

Reassembly: left — installing the name ring with a rubber friction tool; right — the rear mechanism with the coupling pin seated correctly.

What I Learned (The “Why” Summary)

1. Stuck CZJ apertures are extremely common.
2. Heat dramatically increases oil migration.
3. Rear-only cleaning helps mild cases but fails for heavy contamination.
4. IPA temporarily worsens symptoms as oil dissolves.
5. Front access is mandatory when oil is on the blade fronts or in pivot channels.
6. Graphite is the correct long-term dry lubricant for CZJ iris mechanisms.
7. The previous “CLA” clearly used too much grease or the wrong lubricant.
8. Fear and panic are normal during your first iris repair — patience wins.

Conclusion

This repair took me through phases of hope, panic, uncertainty, frustration, and finally relief.
In the end, the Biometar works better than when I bought it.

At the end of the day, everything is ok. If not, it’s not the end of the day. Oh wait, and the answer is always 42!

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© 2025 Henry