Knowledge
Parfocality and Centering Adjustment for Zoom Lenses: A Field Procedure You Can Follow at the Machine
Two mechanical adjustments decide whether a continuous zoom lens is usable in an automated inspection cell: parfocality (the image stays in focus at every magnification) and centering (the optical axis lines up with the centre of the camera sensor). They are independent, they are both done by hand with an Allen key, and they must be performed in that order — parfocality first, centering second. Doing them the other way round destroys the parfocality result, because centering works by loosening and tightening the very C-mount screws that hold the object distance in place.
This procedure is the field version of the manufacturer's adjustment method. It needs a live camera image with a crosshair overlay, a calibration target printed with 0.01 mm grid lines, and a small Allen key. No software, no fixtures and no special tooling.
Key facts at a glance
| Item | Value |
|---|---|
| Adjustment order | Parfocality first, centering second — never reversed |
| Parfocality iterations | Repeat the focus cycle 1–2 times before locking |
| Centering pass criterion | Reference point within a 0.01 mm grid line at both minimum and maximum magnification |
| Centering iterations | 1–2 passes |
| Reference feature | A black grid line 0.01 mm wide or finer on a calibration target |
| Tools required | Crosshair overlay, calibration target, small Allen key |
| Re-adjustment trigger | Any change of camera body requires centering to be redone |
What parfocality actually means
Parfocality means the object distance stays unchanged while the lens zooms: the image must remain in focus at every magnification, from the minimum end of the range to the maximum.
A continuous zoom lens contains several lens groups that move together to compensate for each other. Mechanical and assembly tolerances make the real image plane drift slightly from the theoretical one, which shows up as a lens that goes soft at one particular magnification. Adjusting parfocality mechanically nulls out that drift.
This is where most people go wrong. Parfocality is not "getting the lens sharp". It is removing image-plane drift across the zoom range. A lens that looks sharp at one magnification has not been made parfocal — it has simply been focused.
Parfocality: the five-step procedure
| Step | Action | Why |
|---|---|---|
| 1 | Set the lens to maximum magnification. Adjust the working distance until the image is sharp. | Depth of field is shallowest at maximum magnification, so focus is most sensitive there — that is where you want to establish the reference. |
| 2 | Loosen the focus-ring screws and the C-mount screws slightly, just enough that the ring can be turned. | "Slightly" means only until it moves. Do not remove the screws. |
| 3 | Zoom to minimum magnification. The image will now be blurred. While holding the C-mount firmly against the lens barrel, rotate the focus ring — try both directions and keep the one that sharpens the image — until the image is sharp again. | Holding the C-mount and turning the focus ring are two simultaneous actions. This is the core move of the whole procedure. |
| 4 | Return to maximum magnification and repeat steps 1–3 once or twice, until the image is sharp at both extremes. | One pass is usually not enough. The manufacturer explicitly specifies one to two iterations. |
| 5 | Tighten the focus-ring screws. | If you are going on to centering, leave the C-mount screws loose. |
Deciding whether to continue to centering
Parfocality complete
|
+-- Centering needed? -- No --> tighten focus-ring screws AND C-mount screws -> done
|
+-- Centering needed? -- Yes -> tighten focus-ring screws ONLY,
leave C-mount screws loose -> continue to centeringWhat centering actually means
The beam centre line travelling from the lens to the camera should coincide with the centre line of the camera sensor. In practice it does not, because the sensor is not perfectly centred in the camera body and the beam is not perfectly centred in the lens barrel. The C-mount is the adjustment that brings the beam onto the centre of the sensor.
Mechanically, the C-mount is held by three screws. Tightening and loosening different sides tilts the mounting plane very slightly, which translates the beam centre as projected onto the sensor. In other words, centering compensates for optical-axis error with a controlled mechanical tilt.
Because of this, centering aligns the lens axis to that particular camera's sensor. Change the camera and you must centre again.
Centering: the five-step procedure
| Step | Action | Notes |
|---|---|---|
| 1 | Turn on the camera crosshair overlay. | Most industrial cameras support a crosshair or reticle overlay. |
| 2 | Set the lens to maximum magnification. Move the calibration target until the crosshair centre sits on a chosen reference point, and note that point. | The manufacturer recommends a black grid line 0.01 mm wide or finer. The finer the line, the more precise your judgement. |
| 3 | Zoom to minimum magnification. Before centering, the reference point will visibly drift away from the crosshair centre. | For example, at minimum magnification the reference may sit to the left of the crosshair. |
| 4 | Apply the rule "loosen the side it drifted toward, tighten the opposite one" to the three C-mount screws: loosen the screw on the side the reference point moved toward and tighten the opposite screw, until the reference point returns to the crosshair centre. | Make small adjustments. Do not tighten anything fully in one go. |
| 5 | Return to maximum magnification. If the reference point is still off centre, repeat steps 2–4 once or twice. | Pass criterion: at both minimum and maximum magnification the centre lands within the 0.01 mm line. |
Applying the "loosen the side it drifted toward" rule
| Observation | Action |
|---|---|
| Reference point drifts left | Loosen the screw on the left, tighten the one on the right — this shifts the beam to the right |
| Reference point drifts right | Loosen the right, tighten the left |
| Reference point drifts up or down | Same principle on the corresponding side of the three screws, which are arranged at 120° intervals |
The underlying logic: loosening a screw raises the mounting face on that side, so the beam shifts that way and the reference point comes back to centre.
A manufacturer technique worth following: do not tighten the three C-mount screws firmly in a single pass. Tightening all the way locks out the remaining adjustment travel, and you will have to start over. Bring them up in small increments, checking the crosshair after each one.
Pass criteria and iteration count
| Item | Criterion |
|---|---|
| Reference | At both minimum and maximum magnification, the reference point falls within the 0.01 mm grid line |
| Permitted deviation | 0.01 mm or less |
| Iterations | 1–2 passes |
Common problems and what actually causes them
Parfocality does not hold — the lens goes soft again after a zoom cycle
Four causes, in order of likelihood:
- Not enough iterations. One pass frequently does not converge. Two is the normal requirement.
- The order was reversed. If centering was done first, or interleaved, the C-mount loosening has disturbed the object distance and the parfocality work has been undone.
- The focus-ring screws were not locked, or locking moved the ring. Tightening a screw can shift the ring slightly. Re-check the image after locking, not before.
- The lens is a motorized model that was not homed. Return it to origin first so that the mechanical reference is known, then adjust.
Do I really need a calibration target?
Strongly recommended. The criterion is a 0.01 mm deviation, and printed characters or a general scene simply do not let you judge at that scale. A target with 0.01 mm or finer black grid lines gives you the reference the method assumes.
I changed the camera — do I need to redo this?
Redo the centering, yes. Centering aligns the lens axis to a specific sensor position, and sensor mounting offsets differ between camera bodies. Parfocality is a property of the lens alone and normally survives a camera swap, but re-verify it after any mechanical disturbance.
Is this automatic focus?
No. This is an entirely mechanical, manual procedure carried out by a technician with an Allen key. POMEAS motorized zoom lenses provide powered zoom and powered focus trim; they do not contain a sharpness algorithm or an automatic focus search. See the programmable optics capability breakdown for the exact boundary.
Parfocality and centering — are they the same thing?
No, and they act on orthogonal axes. Parfocality corrects focus drift along the optical axis (Z) as magnification changes. Centering corrects lateral offset (X/Y) between the optical axis and the sensor centre. Fixing one does nothing for the other, which is exactly why both are specified.
Field record sheet
Copy this table into your equipment log. Recording both the iteration count and the final state makes later drift diagnosable.
| Item | Record |
|---|---|
Lens model (PMS-) | |
| Camera model / sensor size | |
| Calibration target line width | |
| Parfocality: working distance at maximum magnification | mm |
| Parfocality: iteration on which it converged | pass ___ |
| Parfocality: focus-ring screws locked | ☐ |
| Centering: reference point at maximum magnification | |
| Centering: reference point at minimum magnification | |
| Centering: deviation within 0.01 mm | ☐ |
| Centering: iteration on which it converged | pass ___ |
| Centering: three C-mount screws locked | ☐ |
| Final verification: sharp and centred at both extremes | ☐ |
Related reading
- Zoom lens parfocality and repeatability FAQ — the optics behind the drift you are correcting
- FA industrial lens selection guide: sensor size, FOV, WD and accuracy — working out the coverage at each magnification
- RS-232 serial command reference — homing the lens before adjustment
- Programmable Optics for Machine Vision: Capabilities and Limits
Product pages
- PMS-LZL-12101D — 12.5X coaxial industrial zoom lens
- PMS-LZG-63104-A — 6.5X motorized zoom rail lens
- 6.5X zoom lens with 3 mm fine focus (0.7X–4.5X, 2/3")
- 6.5X continuous zoom lens with 12 mm powered fine focus (WD 87 mm)
Real integration example: Motorized zoom lens equipment integration: WD, mechanical length and parfocality verified - a documented automation case, not a product spec sheet.
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