Spacer Rings, Varifocal or Custom Focal Length: Choosing and Documenting an Exact FOV
Answer the field of view question with the error budget, not with whichever option is easiest to buy. If a catalog lens already produces...
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About 20 ms. That is the Lensmind ZWA electrowetting number, not a brochure adjective. An autofocus liquid lens in that family changes power by shaping a liquid interface. A voice coil motor (VCM) changes focus by sliding glass. On a 24/7 sorter with mixed package heights, those two mechanisms do not age the same way. The liquid cell has no moving parts. The VCM does.
Key Takeaways
- ZWA electrowetting focus is about 20 ms, with no moving parts.
- VCM autofocus moves a glass group. Wear and heat are the industrial cost.
- ZWA models: M0834AFC, M1238AFC, M1643AFC, H1670AFC-A, H1670AFC-B.
- Polymer-membrane liquid cells driven by a coil are not the same as electrowetting. Do not mix the datasheets.
- If working distance never changes, buy a locked FA lens. Autofocus of either type is wasted motion.
- Ask for the focus method on the RFQ: electrowetting, VCM, or fixed.
Twenty Milliseconds and No Moving Parts
Electrowetting applies a voltage across a liquid interface. The curvature changes. Focal power changes. The glass groups do not translate. That is why the ZWA line can quote about 20 ms and “no moving parts” in the same sentence. A VCM cell cannot. It is a motor.
e-con Systems and the Edge AI and Vision Alliance walk the same split: liquid focus in milliseconds, VCM slower because glass has to move, more heat from friction, worse lifetime on continuous refocus. Those are mechanism facts. They are not a cycle-count from Lensmind. Do not paste a “billion cycles” claim onto ZWA unless it is on the drawing you hold.
If the RFQ says “autofocus” and does not name electrowetting or VCM, you do not yet have a spec.
Liquid Lens vs VCM Compared
| Parameter | ZWA electrowetting | VCM |
|---|---|---|
| Mechanism | Voltage shapes a liquid interface | Coil moves a glass group |
| Moving parts | None | Yes |
| Focus time (ZWA) | About 20 ms | Longer; mass must move |
| 24/7 mixed height | Intended duty | Wear is the risk |
| Fixed working distance | Usually overkill | Usually overkill |
Verdict — electrowetting: Use ZWA when working distance jumps every part and the line never stops. Quote about 20 ms. Do not invent a cycle life.
Verdict — VCM: Acceptable on short-duty embedded modules where cost dominates and refocus is occasional. On a sorter that hunts focus all shift, you are buying a wearing motor.
Electrowetting Is Not a Polymer Coil
“Liquid lens” is a family name, not a single physics. Electrowetting (ZWA) uses voltage on a liquid interface. A different industrial family uses a polymer membrane and a coil to pump liquid. That second family still has an actuator. Mixing those datasheets is how an RFQ quotes 20 ms next to a current-driven membrane cell that was never measured that way.
Name the physics on the purchase order: electrowetting, polymer/VCA, or VCM glass. Then name the model. Lensmind ZWA is M0834AFC, M1238AFC, M1643AFC, H1670AFC-A, H1670AFC-B. If the drawing is one of those, you are in electrowetting. If it is not, do not borrow the 20 ms figure.
HANGZHOU FACTORY · ISO 9001
ZWA Liquid Lenses, About 20 ms, No Moving Parts
Lensmind ZWA electrowetting cells — M0834AFC, M1238AFC, M1643AFC, H1670AFC-A, H1670AFC-B — change focus in about 20 ms without sliding glass. If the sorter sees mixed heights all shift, start here. If working distance is fixed, stay on a locked FA lens.
About 20 ms
No Moving Parts
ISO 9001
When Autofocus Is the Wrong Buy
A gauging nest at one working distance does not need autofocus. Distortion below 0.05% on a locked FA cell is the cheaper, more repeatable path. The machine vision lens selection guide still starts from sensor, distance, FOV, resolution and environment. If distance is a constant, AF is not an input. It is a cost.
| Best for | Not ideal for |
|---|---|
| ZWA on mixed-height 24/7 readers | A single-height gauging nest |
| VCM on occasional-focus modules | A sorter that refocuses every part |
| Locked FA when WD never changes | An RFQ that only says “AF lens” |
If mixed height is real, start from liquid lenses in the ZWA list. If it is not, stay on FA. Do not add a driver board so the station can “hunt” a distance that never moves.
What We Check on a ZWA Build
- Physics named: electrowetting, not a generic “liquid.”
- Model: one of M0834AFC, M1238AFC, M1643AFC, H1670AFC-A, H1670AFC-B — or a custom drawing.
- Working-distance range: min/max the sorter will actually see, not infinity to 50 mm “just in case.”
- Drive: voltage interface and settling that match the about-20 ms claim on that cell.
- Batch report: MTF, distortion, BFL, DOF, pass/fail. Class 10,000 assembly. ISO 9001 from 2024.
1 μm-class active alignment applies to units that demand it. Liquid cells still need centration. A fast focus that is decentered is a fast blur.
Mixed Heights on a Sorter
Illustrative sorter. Boxes from 80 mm to 280 mm tall. A VCM module hunts every box. After months of continuous duty the coil still moves, but the rest position has drifted. Decode rate falls on the short boxes first. Replacing the cell with a ZWA electrowetting unit removes the sliding mass. Focus time is about 20 ms. The mechanical wear path is gone.
The opposite station is a 25 mm FA macro over one nest. Adding AF there inserts a driver, a cable, and a new failure mode for zero extra height range. Leave it locked.
Conclusion
Electrowetting changes power without sliding glass. VCM slides glass. ZWA is about 20 ms with no moving parts. If working distance is mixed and the line never stops, that is the mechanism to buy. If distance is fixed, buy a locked FA lens.
Key Takeaways
- Write “electrowetting ~20 ms” or “VCM” or “fixed” on the RFQ. “AF” is not a spec.
- Do not copy polymer-membrane cycle claims onto ZWA.
- Name the model: M0834AFC through H1670AFC-B, or a drawing.
- Fixed WD still belongs on FA with distortion below 0.05% when gauging.
If the sorter sees mixed heights, send min/max distance and stay on ZWA. First-pass feasibility still returns within 48 hours. If the nest never moves, do not add a motor so the brochure can say autofocus.
Written by Baker Zhao, Founder at Lensmind Optics. Hangzhou factory in the Lin’an optics cluster; ISO 9001 certified 2024. Company registered 2019.
Frequently Asked Questions
How fast is a Lensmind liquid lens?
ZWA electrowetting focus is about 20 ms, with no moving parts. That figure belongs to the ZWA series, not to every product called a liquid lens.
Why avoid VCM on a 24/7 sorter?
VCM moves glass on every refocus. Wear and heat show up as focus drift under continuous mixed-height duty.
Is every liquid lens electrowetting?
No. Some industrial liquid cells use a polymer membrane and a coil. Lensmind ZWA is electrowetting. Name the physics on the RFQ.
Do I need autofocus at a fixed working distance?
No. Use a locked FA or M12 lens. Autofocus of either type adds a driver and a failure mode for no extra height range.
Which ZWA models are in the catalog?
M0834AFC, M1238AFC, M1643AFC, H1670AFC-A and H1670AFC-B. Custom cells follow the 60 working-day prototype-to-MP path.
