300mm Working Distance Zoom Lens: 0.25X Objective at 297 mm

How a 12.5X zoom body plus a 0.25X auxiliary objective reaches roughly 300 mm of standoff as a long working distance zoom lens — and why the field ceiling comes from the sensor.

Two questions decide whether a 297 mm configuration is right. Can the machine get a lens to the part? That is clearance, answered in millimetres of air. Can the optics resolve what you need? That is sampling, and it gets harder the further back you go.

When 77–155 mm Is Not Enough: What ~300 mm WD Buys You

The bare 12.5X body sits at 77.4 ± 2 mm. The 0.5X objective takes it to 155 mm. When a cell needs a long working distance zoom lens instead, the 0.25X objective takes the same body back to 297 mm — the closest catalogue value to a round 300 mm.

That standoff buys no optical advantage. It buys the ability to take the picture at all:

• A gap that already exists. A protective window, sight glass or splash guard.
• Room for illumination. A ring, bar or coaxial light that must touch neither surface.
• Moving material. A web, roll, shuttle or robot arm crossing the field.
• Heat. A part still hot at inspection, and a lens that must stay clear.
• Mechanical interference. Grippers, second tools, fixture walls designed before the lens.

Image quality is not on that list. A 297 mm setup is no sharper than a 155 mm one — it is the same chain pushed back, and the bill arrives below.

The 297 mm Configuration: 12.5X Body + 0.25X Auxiliary Objective

One combination produces 297 mm: the PMS-LZL-12101 body with the 0.25X auxiliary objective (SKU 752025). The body sets magnification, resolution and sensor fit; the objective sets working distance.

Published 12.5X figures: 0.58X–7.5X; working distance 77.4 ± 2 mm with no attachment; sensor up to 2/3-inch; resolution 13 µm at 0.58X improving to 3.05 µm from 4.5X; numerical aperture 0.025–0.11; TV distortion down to 0.001%.

Then the rule that catches people out. With the objective fitted, working distance is locked at 297 mm. Zoom changes magnification and field. The TV tube changes magnification and field. Neither moves the standoff. In the A-class chain, the objective is the only working-distance knob you have.

Figures below are calculated as 0.58X–7.5X × 0.25X × TV factor, with HFOV = sensor width ÷ effective magnification on 8.8 × 6.6 mm; 297 mm is the published working distance.

The TV tube is a 4× lever on field of view at zero standoff cost — the cheapest adjustment here, and the one most often chosen last. Background: what an auxiliary objective changes, and what it leaves alone.

How Wide the Field of View Can Actually Go at 297 mm

Widest is 0.25X objective plus 0.5X TV tube: 0.0725X minimum effective magnification, dividing the 8.8 mm sensor width into 121.4 mm and its height into 91.0 mm (calculated). Zoomed fully in, the stack narrows to 9.4 × 7.0 mm (calculated).

That 121.4 mm is not created by the standoff. It is three hardware floors — 0.58X zoom, 0.25X objective, 0.5X TV tube — applied to an 8.8 mm sensor. Push the standoff further with another front optic and the ceiling does not move, because distance is not in the equation: field of view = sensor width ÷ effective magnification.

So a spec asking for 200 mm of coverage at 300 mm cannot be met here: the arithmetic ceiling is 121.4 mm (calculated). The rules behind that ceiling are covered in the image circle rule in detail.

One family warning: the 0.25X factor does not give 297 mm on every body — the 6.5X/K chart lists 240 mm. The 297 mm figure is 12.5X-only, so quote body and objective together.

The Price of 0.25X: Resolution, Brightness and DOF

A demagnifying objective is a trade, not an upgrade.

Sampling. The 0.25X objective divides effective magnification by four, so a given detail lands on four times fewer sensor pixels in each direction. Published 12.5X resolution scales the same way, being object-side (calculated).

At minimum zoom the 0.25X objective moves the object-side sampling limit from 13 µm to approximately 52 µm (calculated). The table rows are that arithmetic applied across the tiers: published body resolution — 13 µm at 0.58X, 3.05 µm from 4.5X — divided by the objective factor. Real defect visibility also depends on lighting, contrast and pixel sampling. A job needing 20 µm features comfortably should not be planned around this route at minimum zoom.

Brightness. The 0.25X front optic gathers light over a narrower angle than the bare front end, so less light per unit area reaches the sensor even though the field got wider. Budget more illumination than the bare body needed, and expect longer exposure to fight throughput. See how brightness changes with magnification.

Depth of field. POMEAS does not publish depth of field for objective combinations, and the body figure does not transfer — for the 12.5X it is not published per magnification at all. Confirm it for your exact stack: working distance versus depth of field, improving depth of field in a real cell.

297 mm vs 155 mm vs 93 mm: Choosing Your Tier

Three objectives, three standoffs, one body. Pick the tier by the clearance you have, not the field you want — the TV tube sets the field later. Fields calculated at 1X TV on 2/3-inch; working distances are published.

Two things the table hides. Stepping from 0.25X to 0.5X doubles magnification and halves the field at any zoom position. And every tier keeps the same zoom travel, so the band only shifts, never changes shape.

The rule that saves money: if your clearance is 160 mm, do not buy 297 mm for safety. A longer standoff is not a safer lens — it costs sampling and brightness, and buys only air. The middle tier is worked through in 155 mm working distance configurations.

The Magnification Range You Keep at 0.25X

People assume a 0.25X objective kills the zoom range. It does not. It shifts the range down by four times and leaves the travel intact: 0.58X–7.5X becomes 0.145X–1.875X, the same span at a quarter of the size (calculated).

That matters most at 300 mm, because you can still zoom: start wide to find the part, zoom in to read a mark or check a seam, and never move the camera.

Position the band with the TV tube. A 0.5X tube gives 0.0725X–0.9375X and 121.4 → 9.4 mm for large-area scanning. A 1X tube gives 0.145X–1.875X and 60.7 → 4.7 mm as the default. A 2X tube gives 0.29X–3.75X and 30.3 → 2.3 mm when detail matters more than area (calculated, 2/3-inch). A 30 mm target field keeps the long standoff only on that 2X route — compared in 30 mm FOV configurations.

The 2X tube does not buy resolution, though: it magnifies a field the objective already sampled.

Sensor and Image Circle: Why a 1" Camera Is the Wrong Upgrade

If 121.4 mm is not wide enough, instinct says bigger sensor: 1-inch is 12.8 × 9.6 mm, about 176.6 mm of field at 0.0725X (calculated). That 176.6 mm field cannot be collected — the 12.5X body accepts up to 2/3-inch, and a 1-inch sensor vignettes.

So the field ceiling at 297 mm is set by the image circle — the circle of light the lens projects onto the sensor — not the distance.

Upgrading the camera here makes the picture worse. A 1-inch sensor adds dark corners, not field, and the corners are where your part sits. Rules in camera sensor and image circle matching, the failure mode in low magnification vignetting in the corners. If 1-inch coverage is mandatory, the answer is a body built for it, and 297 mm leaves the picture — see the configuration guide.

Long Working Distance Applications: What Justifies 297 mm

297 mm is not a default. These process types show where the reason holds.

• Hot or cooling parts. Castings, weld beads, glass and formed metal still hot at inspection, where standoff keeps optics out of the plume.
• Moving material. Webs, rolls, strip and sheet, where the lens must sit clear of the path.
• Energized assemblies. Battery modules, busbars and high-voltage connectors, where standoff is also a safety requirement.
• Committed fixtures. Cells designed around 250–300 mm before the vision system existed.
• Protective windows and dirty zones. Window thickness eats standoff; foundry, coolant and washdown lines eat front elements.

Where it is wrong: defects at the limits in the sampling table, and cells genuinely 150 mm deep. A 297 mm configuration solves clearance, not resolution. For micron detail the route is a high-magnification APO stack, covered in zoom lenses with APO metallurgical objectives.

A 297 mm Pre-Order Checklist

1. Measure the real gap. Objective front reference to part surface, worst case in the cycle — not the nominal CAD value.
2. Subtract what sits in the gap. Window, film, light guide, nozzle. Anything between objective and part is standoff already spent.
3. Confirm the sensor. 2/3-inch is the largest that works. A 1-inch camera vignettes whatever the arithmetic promises.
4. Check the sampling honestly. Compare your smallest defect with the limits above, then decide whether zooming in gets you there.
5. Choose the TV tube last. Select it against the target field, after the standoff tier is fixed.
6. Decide manual, motorized or rail. At 297 mm the lens sits far from the operator: compare manual versus motorized zoom lenses and the LZLG-12104-A rail body.
7. Ask POMEAS to confirm depth of field for the combination, the objective SKU, and the illumination plan.

Next step: check the LZL-12101 12.5X continuous zoom lens specification, then send POMEAS your clearance, sensor format and smallest defect.

FAQ: Long Working Distance Questions

What lens gives about 300 mm working distance?

The 12.5X continuous zoom body with a 0.25X auxiliary objective: the published working distance for that combination is 297 mm, the closest catalogue figure to 300 mm. No bare zoom body reaches 300 mm on its own.

How do I get a large field of view at a long working distance?

Stack the lowest objective factor with the lowest TV factor. A 0.25X objective plus a 0.5X TV tube gives 0.0725X minimum effective magnification and a 121.4 mm horizontal field on a 2/3-inch sensor, calculated from the 8.8 mm sensor width.

What does a 0.25X auxiliary objective do to magnification and FOV?

It divides effective magnification by four at every zoom position and widens the field four times, while fixing the working distance at 297 mm. Object-side sampling also quadruples, so the same detail is imaged more coarsely than at 155 mm.

Why does a bigger sensor not give a wider field of view here?

Because the 12.5X body accepts sensors up to 2/3-inch only. A 1-inch sensor would calculate to about 176.6 mm of field at 0.0725X, but its corners fall outside the image circle and vignette, so that extra width cannot be used.

Is 297 mm the right choice for hot or moving targets?

Yes, when standoff is the real constraint. Hot parts, moving webs, energized assemblies and cells with a protective window are the classic reasons to move back to 297 mm. If the part is small and static, 155 mm usually samples better.

What do I lose by choosing the 0.25X route?

Object-side sampling, image brightness and field-of-view headroom. At minimum zoom the object-side limit moves from 13 µm to approximately 52 µm (calculated), and the reduced aperture asks for more illumination than the bare body needs.

Related: the LZL-12101 coaxial zoom body