Telecentric Lens Sensor Size vs Image Circle Matching: What to Read Before You Buy

A telecentric lens is designed around a maximum sensor format. That single line on the datasheet decides whether the corners of your image are usable, and it is the line most often read backwards: engineers pick the magnification and working distance first, then discover that the lens they have chosen covers a smaller sensor than the camera they already own.

The failure mode is quiet rather than dramatic. A lens that does not fully cover the sensor still produces an image — one with dark corners, falling resolution toward the edge, and a magnification that is no longer the published value away from the centre. On a measurement station those are three separate error sources that all look like "the lens is not accurate".

The second trap is arithmetic. Field of view is sensor width divided by magnification — correct, but only inside the covered circle. Applied to a sensor larger than the lens covers, it gives a number that is geometrically right and practically useless.

Why the image circle is a hard limit rather than a soft one

In a telecentric lens the image-side chief rays run parallel to the axis, so illumination and ray angles at the sensor are uniform across the covered area. That uniformity is bought with glass: the front element has to be at least as large as the object field. Beyond the design circle no light is delivered, which is why vignetting on an undersized lens is abrupt rather than gradual.

This is also why matching is less forgiving on telecentric lenses. A conventional lens asked to cover a slightly larger sensor shows gentle corner falloff that can be tolerated or calibrated out; a telecentric lens shows a defined edge to the illuminated circle.

The six checks for matching a sensor to a telecentric image circle

1. Read the maximum supported sensor format, and treat it as a ceiling

Every POMEAS telecentric product page states a maximum supported sensor, and the camera's format has to be at or below it. Values published across the line include 1/2", 2/3", 1", 1.1", 4/3", 4" and 35 mm (Film), so the ceiling cannot be assumed from the magnification.

PMS-LTCM037-150-P publishes 1.1" at 0.37× and 150 mm; LTCM02-180-G publishes 1"; PMS-LTC2-110C publishes 2/3"; PMS-LTC08-90 publishes 1/2". The same magnification can appear at several ceilings, so the sensor line has to be read for the exact model.

2. Read the published field of view at your sensor, not at the maximum

Several POMEAS models publish a field-of-view table giving the field for each supported sensor format. Where that table exists it is the number to use, because it is the designed value for that pairing rather than a division you performed yourself.

LTCM02-180-G publishes, at 0.2× and 180 mm working distance:

The product page marks these figures as approximate: actual field depends on the camera's effective photosensitive area, the mounting state and the commissioning conditions. Two cameras both sold as "1-inch" need not have identical active areas.

3. Confirm the mount before the format

Large-format telecentric lenses do not use C-mount. The POMEAS F-mount bi-telecentric line covers the biggest sensors: PMS-LTFM0106-468 publishes 4/3" at 0.106× and 468 mm, PMS-LTFM017-416 publishes 35 mm (Film) at 0.17× and 416 mm, and PMS-LTFM1-127 publishes 35 mm (Film) at 1.0× and 127 mm, explicitly including 4K/8K line-scan cameras.

If the camera has an F-mount or a large-format sensor, the C-mount telecentric range is not the place to look, regardless of how well the magnification and working distance match.

4. Check whether the model publishes a multi-format field table or a single value

Some models publish one field of view for one sensor; others publish a table. PMS-LTCM2-65-S publishes four — 1/4" at 1.6 mm × 1.2 mm, 1/3" at 2.4 mm × 1.8 mm, 1/2.5" at 2.9 mm × 2.2 mm and 1/2" at 3.2 mm × 2.4 mm — at 2.0× and 65 mm, maximum 1/2".

PMS-LTC2-110C publishes five entries including 1/1.8" at 3.6 mm × 2.7 mm, and states support from 1/4" through 2/3". A stated range is more useful than a single maximum, because it shows the lens was evaluated at both ends.

5. Check the resolution figure against your pixel pitch, in object space

The published object-side resolution is an optical figure. Whether the camera can record it depends on the pixel pitch divided by the magnification — the pixel's footprint projected back onto the part. If that footprint is larger, the camera is the limit; if much smaller, the lens is.

POMEAS telecentric models publish object-side resolution figures spanning 1.1 µm (PMS-LGX8-95, 8.0×) through 87.99 µm (PMS-LTCM0061-398, 0.061×). Matching one of those to a pixel pitch is arithmetic the buyer has to do with the camera's own datasheet.

6. For line-scan, check the row field and the supported line length

Area cameras and line-scan cameras stress the image circle differently. PMS-LTFM017-416 publishes a line-scan 8K row field of 241.2 mm (8K × 5 µm) alongside its Φ254 mm object field, and PMS-LTFM1-127 states support for 4K/8K line-scan cameras.

For line-scan work the row field is the specification to match; the diagonal coverage argument used for area sensors does not transfer directly.

Where POMEAS telecentric lenses land on those six

Published values across the common sensor formats:

Reading the table against the six checks:

- For a 1/2-inch camera: PMS-LTC1-65, PMS-LTC08-90 and PMS-LTCM2-65-S all publish 1/2" as the maximum, and all three publish a field-of-view table. - For a 2/3-inch camera: PMS-LTC2-110C publishes 2/3" as its maximum and is the only model here publishing a 1/1.8" entry. - For a 1-inch camera: LTCM02-180-G carries the four-step field table up to 64.0 mm × 48.0 mm, but publishes no resolution figure. - For a 1.1-inch camera: PMS-LTCM037-150-P publishes 1.1" at 0.37× and 150 mm with 11.18 µm resolution and 3.6 mm depth of field, but no field-of-view table. - Above 1.1 inch, the F-mount bi-telecentric range applies: PMS-LTFM0106-468 at 4/3", PMS-LTFM017-416 and PMS-LTFM1-127 at 35 mm (Film), and PMS-LTCM0063-416-G at 4".

Each row carries the fields that model publishes; a blank is a gap in published data, not a value to be carried across.

What those numbers mean on the line

Scenario 1 — A 1-inch camera on a lens specified for 2/3 inch

The station reads corner dimensions inconsistently and the corners are darker. PMS-LTC2-110C publishes 2/3" as its maximum, so a 1-inch sensor is outside the design circle. The fix is a lens whose published maximum is at least 1 inch — LTCM02-180-G, at the cost of a different magnification and working distance.

Scenario 2 — Choosing between two 1/2-inch lenses at the same magnification

PMS-LTCM2-65-S and PMS-LTC2-110C are both 2.0× with a 1/2" field of 3.2 mm × 2.4 mm published, and differ in working distance (65 mm against 110 mm), resolution (2.2 µm against 8 µm) and maximum sensor (1/2" against 2/3"). With a 1/2" camera the ceiling is not the deciding factor.

Scenario 3 — A 1.1-inch camera on a compact station

PMS-LTCM037-150-P publishes 1.1" support at 0.37× with 150 mm working distance, 11.18 µm resolution and 3.6 mm depth of field, in a lens 259.2 mm long. The ceiling is met; the length is what has to be checked against the envelope.

Scenario 4 — A line-scan camera on a large part

PMS-LTFM017-416 publishes a 241.2 mm row field for 8K × 5 µm line-scan cameras with a Φ254 mm object field at 0.17× and 416 mm working distance, and 22.14 mm of depth of field. For a web or a large panel, the row field and the 8K support decide the match.

What this article does not tell you

This is a selection reference, not a compatibility statement. No claim is made that any POMEAS telecentric lens is a drop-in replacement for a specific third-party lens on a specific camera, and none should be inferred. Whether a given pair works depends on the mechanical interface, the flange distance, the sensor's active area and the illumination.

Optical coverage is not measurement accuracy. A lens that fully covers the sensor delivers light to every pixel; measurement 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 — field of view for PMS-LTCM037-150-P, resolution for LTCM02-180-G, magnification and working distance for PMS-LTC1-65 — read it as "not published".

The telecentric content already on this site covers the four-number selection procedure, the per-magnification model families and 1.1-inch camera matching as separate pages.

FAQ

What happens if my camera sensor is larger than the lens covers?

The outer part of the sensor receives little or no light. Corners darken, resolution falls toward the edge, and any dimension measured near the edge becomes unreliable. It is a hard limit, not a gradual degradation you can calibrate away.

Can I compute the field of view from sensor width divided by magnification?

Inside the covered circle, yes. Outside it, the arithmetic is correct but the result is not usable, because that part of the field is not being imaged. Where the model publishes a field table, use the table.

Which POMEAS telecentric lenses cover a 1-inch sensor?

LTCM02-180-G is published with a maximum of 1 inch and a four-step field table up to 64.0 mm × 48.0 mm. Above that, the F-mount bi-telecentric line covers 4/3 inch and 35 mm (Film) formats.

Does a larger sensor always give a larger field of view?

Within one lens, yes — the published tables for LTCM02-180-G run from 24.0 mm × 18.0 mm at 1/3 inch to 64.0 mm × 48.0 mm at 1 inch. Across different lenses, magnification dominates and the comparison has to be made model by model.

Why do two cameras both sold as 1 inch give different fields?

Because the active photosensitive area is not identical between models. The LTCM02-180-G page states this explicitly and marks its field figures as approximate, with actual field depending on the camera's effective area and mounting state.

What should I check for a line-scan camera instead of sensor format?

The row field and the supported line length. PMS-LTFM017-416 publishes 241.2 mm for 8K × 5 µm, and PMS-LTFM1-127 states support for 4K/8K line-scan cameras.

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.