Telecentric Lens Working Distance: Why It Is Fixed and How to Choose It

On a conventional lens, working distance is where you happened to focus. On a telecentric lens it is where the lens was designed to work, published as a single number for a reason: the telecentric condition holds at one object plane. Move away from it and magnification stays nominally constant only within the depth of field.

That number then becomes the constraint everything else has to fit around: the lighting, the fixture, the gripper, the conveyor edge and any guarding. And the lens itself has to fit behind it — a telecentric lens with a long working distance is physically long, because the front element has to be at least as large as the object field.

Choosing a working distance is therefore not an optical optimisation. It is a clearance calculation followed by a shortlist. The mistake worth avoiding is picking the longest distance available on the assumption that more room is safer: longer working distances come with larger lenses and coarser published resolution.

Why telecentric working distance is a designed value

In an object-side telecentric lens the aperture stop sits at the front focal plane, so chief rays in object space run parallel to the axis. That geometry is designed for one object plane; the published depth of field is how far the part may move around it while the image stays sharp — a tolerance, not a focus range.

This is why the two numbers must be read together. A lens with a 398 mm working distance and 171.97 mm of depth of field absorbs a very different production reality from one with 9.5 mm and 0.017 mm, although both are "a telecentric lens".

The six checks when choosing a telecentric working distance

1. Establish the minimum clearance first

Working distance is bounded from below by mechanics, not by optics. What has to fit between the front of the lens and the part — a ring light, a conveyor edge, a gripper approaching from above, a fixture wall, a guard, or a part taller at one end of its travel?

Measure that clearance with the station built, not on a drawing. Then shortlist models whose published working distance exceeds it with margin. Every other consideration comes second, because a lens that cannot physically see the part is not a candidate at any price.

2. Read working distance and depth of field as two separate numbers

Working distance is where the design works. Depth of field is how much axial error the part may have around that point. Published POMEAS pairs:

The depth of field column is not a function of the working distance column — it is set by the aperture and the design, and read per model. PMS-LTCM02-110 publishes 10 mm at 110 mm, the configuration intended for parts at varying heights on a line.

3. Understand that working distance does not set the field of view

Field of view is set by magnification and sensor size:

FOV = sensor width / magnification

A longer working distance does not by itself give a larger field; it changes which magnifications are available, and the long-working-distance models in the POMEAS catalogue are predominantly low-magnification. PMS-LTCM0061-398 is 0.061× at 398 mm; PMS-LTFM0106-468 is 0.106× at 468 mm with a Φ210 mm object field. If you need long working distance and high magnification together, the catalogue gets thin quickly.

4. Check the lens length, not just the working distance

The total envelope is the lens length plus the working distance plus clearance behind the camera. Published lengths across the line:

- PMS-LTCM0088-120 — 240 mm, Φ116 mm - PMS-LTCM037-150-P — 259.2 mm - PMS-LTCM016-189 — 289.09 mm - PMS-LTFM1-127 — 297.35 mm - PMS-LTFM0106-468 — 558 mm, Φ238 mm - PMS-LTFM017-416 — 712 mm

A 468 mm working distance with a 558 mm lens body is over a metre of envelope before the camera is counted. That is frequently the reason a long-working-distance design is rejected on an existing machine.

5. Confirm the illumination fits inside the working distance

Backlights need their own standoff. The POMEAS VP-GLTD series publishes beam diameters of 30 mm, 60 mm, 100 mm, 145 mm, 189 mm and 265 mm with working distances covering 45–500 mm, and per-model recommended ranges of 45–90, 90–180, 150–300, 100–300, 250–500 and 150–500 mm.

A collimated backlight placed outside its recommended range will not fill the field evenly, and the edge definition it is supposed to provide disappears. On short working distances the backlight is often the binding constraint rather than the lens.

6. Do not maximise working distance by default

Extra working distance costs resolution and size: PMS-LGX8-95 publishes 1.1 µm at 9.5 mm, PMS-LTC05-40 6.7 µm at 40 mm, PMS-LTCM02-110 16.78 µm at 110 mm, and PMS-LTCM0061-398 87.99 µm at 398 mm. Specifying more distance than the mechanics require generally moves the station to a coarser resolution figure.

The discipline is to specify the shortest working distance that clears the mechanics with margin, then look for the best resolution and depth of field combination at that distance.

Where POMEAS telecentric lenses land on those six

Reading the table against the six checks:

- PMS-LGX8-95 is the short-distance extreme at 9.5 mm: NA 0.3, resolution 1.1 µm, depth of field 0.017 mm, M12 mount, maximum 1/3-inch sensor. Nothing fits between lens and part, so the station is built around the lens. - PMS-LTC05-40 at 40 mm with 6.7 µm and 1.6 mm is the compact general-purpose option, publishing 12.8 mm × 9.6 mm on a 1/2-inch sensor. - PMS-LTCM02-110 at 110 mm publishes 10 mm of depth of field and 16.78 µm resolution with TV distortion 0.02% — the model for parts at genuinely different heights. - PMS-LMC03-185 at 185 mm publishes F 6.26, NA 0.024 and TV distortion 0.02%, but no depth of field or resolution. - PMS-LTCM0061-398 at 398 mm publishes 171.97 mm of depth of field, 87.99 µm resolution, TV distortion 0.01% and object-side telecentricity <0.02°. - PMS-LTFM0106-468 at 468 mm is the longest here, with a Φ210 mm object field, 34.95 µm resolution, centre MTF greater than 60% at 70 lp/mm and a 558 mm body at Φ238 mm on an F mount.

Each row lists only what that model publishes. Blank entries are gaps in published data, not zeros, and no value should be carried across from another row or from the model's name.

What those numbers mean on the line

Scenario 1 — A station with 60 mm of mechanical clearance

The gripper needs 60 mm above the part, so the shortlist starts at 65 mm. PMS-LTCM02-110 at 110 mm is the natural candidate, with 10 mm of depth of field covering height variation and 16.78 µm resolution. A 398 mm model would add roughly 290 mm of unnecessary envelope and move the resolution to 87.99 µm.

Scenario 2 — Micro-feature gauging on a rigid bench

The part sits on a precision stage with micrometre repeatability and nothing needs to fit above it. PMS-LGX8-95 at 9.5 mm working distance publishes 1.1 µm resolution and NA 0.3. Its 0.017 mm depth of field is not a problem here because the stage, not the part, defines Z.

Scenario 3 — Robot-presented large parts

Placement repeatability is tens of millimetres and the part is large. PMS-LTCM0061-398 publishes 171.97 mm of depth of field at 398 mm with 87.99 µm resolution: the depth of field removes the placement problem, and the resolution figure then decides whether the smallest feature is measurable.

Scenario 4 — A backlight that cannot be placed where it needs to be

The lens wants 110 mm and the collimated backlight needs its own standoff behind the part. The VP-GLTD series publishes working distances from 45 mm to 500 mm, with the 60 mm beam recommended at 90–180 mm and the 100 mm beam at 150–300 mm. If the frame will not allow that standoff, the edge contrast target has to be revisited first.

What this article does not tell you

This is a selection reference, not a diagnosis of your station. No claim is made about which working distance your application requires. That depends on the mechanical envelope, the part's Z repeatability, the illumination geometry and the algorithm.

Optical performance is not measurement accuracy. Working distance and depth of field are optical and mechanical specifications; accuracy additionally depends on pixel sampling, contrast, edge definition, illumination stability, and the algorithm.

Every figure here is the published value for the specific POMEAS model named beside it, not measured under a single common condition. Where a field is absent — depth of field and resolution for PMS-LMC03-185, lens length for four of the six — read it as "not published".

The telecentric content already on this site groups models by working distance, covering 65 mm, 110 mm, 150 mm and 200 mm families. This article is about the reasoning; those pages carry the model lists.

FAQ

Why is the working distance on a telecentric lens a fixed number?

Because the telecentric condition is designed for one object plane. The aperture stop sits at the front focal plane so chief rays run parallel to the axis, and that geometry holds at the published working distance.

Is working distance the same as depth of field?

No. Working distance is where the design works; depth of field is how far the part may move around that plane while staying acceptably sharp. PMS-LTCM02-110 publishes 110 mm and 10 mm respectively.

Does a longer working distance give a larger field of view?

Not directly. Field of view is sensor width divided by magnification. Long-working-distance models tend to be low-magnification designs, which is why they cover larger fields, but the magnification is what sets the field.

Should I choose the longest working distance available?

No. Extra distance costs resolution and size. PMS-LTCM0061-398 publishes 87.99 µm at 398 mm against 16.78 µm for PMS-LTCM02-110 at 110 mm. Specify the shortest distance that clears the mechanics with margin.

Why is the lens body so long on long-working-distance models?

The front element has to be at least as large as the object field. PMS-LTFM0106-468 is 558 mm long at Φ238 mm diameter for a Φ210 mm object field at 468 mm working distance.

Does the backlight need its own working distance?

Yes, and it is often the binding constraint. The VP-GLTD series publishes beam diameters from 30 mm to 265 mm with working distances from 45 mm to 500 mm, and each beam diameter has its own recommended range.

Next Step

If you are specifying a replacement and need measured values for your own conditions — your sensor size, your working distance, your feature size — send us those four numbers and we will run them against the current catalogue and tell you what actually fits, including when nothing does.