Alternatives to the C-Mount Lens on a Basler Industrial Camera: 6 Checks

Within the POMEAS global company intelligence set of 2,602 companies, 42 have public material indicating Basler industrial cameras in use. That matters here for one reason: these cameras span every sensor format found on factory floors — 2/3-inch, 1-inch and larger — on the same C-mount thread. The thread is why substitution looks easy, and why it goes wrong. Two lenses that screw into the same flange can differ in image circle, in back focal length, and in whether they focus at all at the working distance the fixture allows.

A C-mount camera also fixes numbers before the lens is chosen. The standard defines a single flange-to-image-plane distance; the camera datasheet states it, and the lens's back focal length has to sit inside the focus range that distance leaves. That value belongs to the camera, not to the lens catalogue.

The six parameters to verify before substituting

The first three eliminate lenses that cannot form an image; the last three eliminate lenses that fail in service.

1. Sensor format first, then the lens image circle

The order is not arbitrary: the sensor is fixed and published, the lens is the variable, so the comparison runs from the camera outward. Establish the active area, then require the lens's published image circle to meet or exceed its diagonal.

POMEAS states coverage two ways — as format, active area and circle (2/3-inch as 8.8 × 6.6 mm with an 11 mm circle, 1-inch as 12.8 × 9.6 mm with a 16 mm circle, 4/3-inch with a 22 mm circle), or as a single diameter, as on the 1.1-inch series at 17.6 mm. A smaller circle vignettes; a larger one is unused.

2. Focal length, from field of view and working distance

FA lenses carry no magnification figure, so focal length is derived:

focal length ≈ sensor size × working distance / FOV

Both this and its rearrangement are thin-lens approximations, for shortlisting rather than for a measurement specification. Confirm the shortlist against the published angles: PMS-LFEH-1616M(A) gives 38.0° × 30.8° × 23.4° on 2/3-inch.

3. Distortion, read at the format the camera actually has

Distortion is published per format at a stated image height, and the two formats you are most likely choosing between give different answers for one lens. PMS-LFHHH-3520M is −0.56% @ y=11.0 mm on 4/3-inch, −0.43% @ y=8.0 mm on 1-inch and −0.23% @ y=5.5 mm on 2/3-inch: one optic, three numbers, the smallest on the smallest sensor. On 1-inch, PMS-LFGH-3514M is 0.27% @ y=8.0 mm while PMS-LFGH-1216M publishes nothing — "not published" is a finding, not a gap to fill from a sibling.

4. Back focal length in air, and the C-mount mechanical constraint

Back focal length (后焦距) is the distance from the lens mounting face to the image plane, measured in air. It decides whether the rear of the lens clears the camera's front plate and any filter glass, and where the focus range lands once the lens is seated.

Because the C-mount flange distance is fixed by the standard, a substitute whose back focal length differs shifts where the focus range falls. Published values run from 10.3 mm in air (mechanical 10.08 mm) on PMS-LFPHM-5028M(B), through 12.36 mm on PMS-LFEH-1616M(A) and 12.83 mm on PMS-LFGH-3514M, to 18.65 mm on PMS-LFHHH-3520M. Compare it with the flange distance from the camera datasheet.

5. Aperture range and closest imaging distance

Aperture range sets the exposure and depth-of-field envelope; closest imaging distance decides whether the lens focuses on your fixture. The apertures run F1.4–F22 to F2.8–F16, and the distances appear in two conventions that must not be converted into each other: a closest imaging distance (0.2 m on PMS-LFEH-1616M(A)) versus a focus or imaging range (0.3 m to infinity on PMS-LFGH-3514M, 1 m to infinity on PMS-LFGH-1216M). A lens starting at 1 m will not focus at 0.4 m.

6. Mount detail, filter thread, weight and operating temperature

C-mount is the interface throughout, and the 20 MP PMS-LFPHM series states the thread as C-Mount, 1-32UN-2A. Filter thread is not consistent across a series: M37.5×0.5 on PMS-LFEH-1616M(A), M39.0×0.5 on PMS-LFGH-3514M, M40.5×0.5 on PMS-LFHHH-3520M, M30.5×0.5 on PMS-LFEHH-1218M(D), not published on PMS-LFPHM-5028M(B). Weight matters on a moving mount — 118 ± 5 g on the 50 mm 20 MP model. Temperature is −10 °C to +50 °C on most of the line, −30 °C to +70 °C on the 1.1-inch 20 MP series.

Writing a replacement specification when the original lens is discontinued

Specify the function, not the part number: sensor format and active area, field of view at the available working distance, the focal length that follows, largest acceptable distortion with its image height, closest imaging distance, flange distance and rear clearance, filter thread, temperature band, and whether vibration makes focus and aperture locking mandatory.

Mark each line must-hold or preferable — coverage, focus range and back focal length are must-hold, aperture range and weight usually preferable. Require the supplier to answer at your format: a distortion claim without a percentage, a format and a y-value cannot enter an uncertainty budget.

Where POMEAS optics land on those six

The 2/3-inch and 1-inch pairs are the ones to read side by side. PMS-LFEH-1616M(A) is the 2/3-inch 16 mm part at −0.07% @ y=5.5 mm, low enough that geometry rarely limits the station; PMS-LFGH-1216M is its 1-inch wide-angle counterpart at 68.8° × 57.1° × 44.1°, but publishes no distortion and starts at 1 m, so it is a large-field, long-distance part.

PMS-LFGH-3514M is the 1-inch choice for a station that measures: 22.7° × 18.2° × 14.1°, 0.27% @ y=8.0 mm, from 0.3 m. PMS-LFHHH-3520M is the answer when one specification has to serve several cameras, publishing angles and distortion for 4/3-inch, 1-inch and 2/3-inch; its 18.65 mm back focal length is the longest here. PMS-LFPHM-5028M(B) and PMS-LFEHH-1218M(D) bracket the close-range case: a 20 MP 1.1-inch 50 mm part from 0.25 m covering approximately 69 × 55 × 41 mm at the close limit on 1-inch, against a 10 MP 2/3-inch 12 mm part from 0.1 m at F1.8. The 25 mm model in that 20 MP series publishes a 6.2 mm C-mount protrusion while the 50 mm does not, which is why rear clearance is confirmed model by model. Figures in different rows are measured on different formats and image heights and are not to be read as if obtained under one set of conditions.

What those numbers mean on the line

Scenario 1 — 2/3-inch camera, fixed fixture, presence and position checking

Coverage and focus range dominate; distortion barely enters. PMS-LFEH-1616M(A) at 16 mm gives 38.0° × 30.8° × 23.4° on 2/3-inch from 0.2 m, and its −0.07% @ y=5.5 mm means the station can take on measurement later.

Scenario 2 — 1-inch camera on a station originally built for 2/3-inch

A sensor upgrade is where corner darkening appears, and the cause is always coverage. PMS-LFGH-3514M covers 12.8 × 9.6 mm against a 16 mm circle and publishes geometry for that format. The trap is the focus range: it starts at 0.3 m, so a fixture sitting closer needs a different part.

Scenario 3 — One lens specification serving several cameras

Mixed sensor sizes on one line are usually handled by stocking several part numbers. PMS-LFHHH-3520M offers an alternative, publishing angles and distortion for three formats — 36.6° × 29.6° × 22.4° on 4/3-inch, 25.9° × 20.8° × 15.7° on 1-inch, 17.9° × 14.4° × 10.8° on 2/3-inch. The trade-offs are its 18.65 mm back focal length and 0.15 m closest imaging distance.

Scenario 4 — Resolution upgrade on a large-format sensor

A 20 MP camera needs a 20 MP-class lens or the extra pixels record nothing. PMS-LFPHM-5028M(B) is rated 20 MP on 1.1-inch at 17.6 mm, focuses from 0.25 m and publishes 0.05% @ y=16 mm — the lowest figure here, making the lens's geometric contribution a minor term in the uncertainty budget. At 118 ± 5 g it is the lightest 50 mm here.

What this comparison does not tell you

This is a parameter reference, not a compatibility statement. No claim is made that any POMEAS FA lens is a drop-in replacement for the lens fitted to a Basler camera, and none should be inferred. Whether a substitute works depends on the camera's flange distance, front recess depth, any filter glass in the path, the illumination and the algorithm's tolerance.

Basler appears here only as recorded in customer-supplied intelligence: within the POMEAS global company intelligence set of 2,602 companies, 42 have public material indicating Basler industrial cameras in use. No Basler specification is quoted, because none was independently verified.

Optical resolution is not measurement accuracy. Resolution is set by the lens, the wavelength and the sensor pitch; accuracy additionally depends on contrast, edge definition, illumination stability, calibration and the algorithm. A 20 MP rating is a design target for a sensor class, not a promise about the smallest dimension a station can hold.

For the generic method — choosing a C-mount lens from scratch, the focal length formula and the full FA series matrix across 2/3-inch, 1-inch, 4/3-inch and 1.1-inch — see the existing POMEAS article on C-mount lens selection for industrial cameras.

FAQ

Does any C-mount lens fit a C-mount camera?

Mechanically yes, optically no. The thread fixes the seating; it does not fix the image circle, the back focal length or the focus range.

In what order should I check the parameters?

From the camera outward: sensor format and active area, lens image circle, focal length from field of view and working distance, back focal length against the flange distance, then closest imaging distance and aperture.

Why does one lens quote three distortion figures?

Because distortion is measured at a stated field height. PMS-LFHHH-3520M is −0.56% @ y=11.0 mm on 4/3-inch, −0.43% @ y=8.0 mm on 1-inch and −0.23% @ y=5.5 mm on 2/3-inch.

Where do I get the number to compare back focal length against?

The camera's flange distance, from the camera datasheet. POMEAS publishes the lens side per model — 12.36 mm on PMS-LFEH-1616M(A), 18.65 mm on PMS-LFHHH-3520M.

My calculated focal length is not an available model. What now?

FA lenses come in discrete focal lengths. Move the working distance to land on an available one, or accept a slightly different field of view.

How do I write a spec when the original lens is discontinued?

State the function: sensor format and active area, field of view at the available working distance, focal length, maximum distortion with its image height, closest imaging distance, flange distance and rear clearance.

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.