Alternatives to Computar C-Mount FA Lenses: 6 Parameters to Verify

Within the POMEAS global company intelligence set of 2,602 companies, 12 have public material indicating Computar C-mount FA lenses in use. This is a different substitution case from replacing a lens on a camera you never specified: when the original part is a lens, the barrel usually carries the focal length, the maximum format and the aperture range. That is a better start than most, and it still is not enough.

Three of the numbers that decide whether the substitute behaves like the original are not marked on any barrel — image circle, back focal length and distortion — yet they decide whether the corners stay lit, whether the focus ring reaches the working distance, and whether the existing calibration survives. Filter thread brings the most annoying failure: the lens fits, focuses and images perfectly while the polariser no longer screws on.

The six parameters to verify before substituting

Read them off the old lens where possible and demand them from the new one where not. Coverage and geometry come first, mechanical fit last.

1. Focal length and the maximum format the original was built for

Focal length determines field of view only with the sensor and the working distance; the maximum format tells you how large a sensor the lens was corrected for, and is the piece most likely to be missing.

FOV ≈ sensor size × working distance / focal length

This is a thin-lens approximation, not a measured value: it ignores principal-plane separation and focus extension. Confirm the shortlist against published angles. PMS-LFHHH-1620M gives 61.0° × 47.2° with a 73.0° diagonal on 4/3-inch, 44.2° × 33.6° on 1-inch and 31.0° × 23.4° on 2/3-inch: one lens, three fields.

2. Image circle against the sensor diagonal

Once the camera's active area is known, the check is a comparison: the lens's published image circle has to meet or exceed its diagonal. POMEAS states coverage 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 12 MP series quoted at 23 mm. Overshoot is harmless; undershoot vignettes. Two POMEAS series sold under the same 4/3-inch name quote 22 mm and 23 mm, so compare the millimetre figure, not the format name.

3. Distortion, at the format and image height you use

Distortion is published per format at a stated image height, and one lens gives a different answer on every sensor it is used with. PMS-LFHHG-5012M(B) is −0.13% @ y=11.5 mm on 4/3-inch, −0.05% @ y=8.0 mm on 1-inch and −0.01% @ y=4.0 mm on 1/2-inch. PMS-LFHHH-1620M, a wide-angle 16 mm, is −2.81% @ y=11.0 mm on 4/3-inch, −1.92% @ y=8.0 mm on 1-inch and −1.06% @ y=5.5 mm on 2/3-inch — an order of magnitude larger by optical design, not by quality, and a figure at y=5.5 mm measures a different part of the field from one at y=11.5 mm.

4. Aperture range and closest imaging distance

Aperture range governs the exposure and depth-of-field envelope; closest imaging distance governs whether the lens focuses on your fixture. Here the apertures run F1.4–F22 to F2.8–F16, and the distances 0.1 m, 0.3 m, 0.9 m and a 1 m to infinity imaging range.

Read the distance exactly as published: PMS-LFGH-2516M states 1 m to infinity; PMS-LFEHH-5028M(B) 0.3 m to infinity; PMS-LFEH-3514(A) 0.3 m; PMS-LFEH-7520M(A) 0.9 m.

5. Back focal length in air

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 substitute clears the camera's front plate and any filter glass, and where the focus range lands once seated.

Published values here are 12.69 mm on PMS-LFGH-2516M, 12.82 mm on PMS-LFEH-3514(A), 20 mm on PMS-LFHHG-5012M(B) and 21 mm on PMS-LFHHH-1620M — wide enough that a swap at the same nominal focal length can move the focus range out of reach. The comparison has two halves: the lens figure from the lens datasheet, the flange distance from the camera datasheet.

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

C-mount is the interface throughout. Filter thread is not, and it is the detail that breaks an optically correct swap: M37.5×0.5 on PMS-LFEH-3514(A), M33.7×0.5 on PMS-LFGH-2516M, M58×0.75 on PMS-LFHHH-1620M, not published on PMS-LFHHG-5012M(B). Confirm it before ordering.

Weight also differs, which matters where the camera moves. Temperature is −10 °C to +50 °C on several models and −20 °C to +60 °C on the 12 MP PMS-LFHHG series; two models here publish none.

Writing a replacement specification when the original lens is discontinued

Specify the optical function, not the part number: sensor format and active area, field of view at the available working distance, worst acceptable distortion with its image height, closest imaging distance, mount and rear clearance, filter thread, maximum barrel weight the bracket accepts, temperature band, and whether vibration makes locking mandatory.

Mark each line must-hold or preferable — coverage, focus range, back focal length, filter thread and distortion are must-hold on most stations; aperture range and weight usually preferable. Add one line often skipped: if the original datasheet is lost, state the resolution class the camera needs and require the substitute to be rated for it. A 5 MP lens on a 12 MP camera is a silent downgrade.

Where POMEAS optics land on those six

The two 2/3-inch 5 MP models sit at opposite ends of the working-distance range. PMS-LFEH-3514(A) at 35 mm covers 17.9° × 14.3° × 10.8°, focuses from 0.3 m and publishes −0.07% @ y=5.5 mm — low enough to measure without a correction model. PMS-LFEH-7520M(A) at 75 mm narrows to 8.4° × 6.7° × 5.0°, starts at 0.9 m and weighs 223 g; with nothing published on distortion or back focal length, it suits inspection rather than gauging.

PMS-LFGH-2516M is the 1-inch option and the only model here publishing resolving power — 160 lp/mm centre, 100 lp/mm periphery — and its −2.06% @ y=8.0 mm on 1-inch is typical for a 25 mm design.

The two 50 mm models are the geometry answers: PMS-LFEHH-5028M(B) at 0.094% @ y=5.5 mm, PMS-LFHHG-5012M(B) down to −0.01% @ y=4.0 mm with a 20 mm back focal length and object coverage of approximately 118.9 × 95.1 × 71.3 mm at its 0.3 m close limit. PMS-LFHHH-1620M is the cautionary wide angle: 4/3-inch from 0.1 m, angles for three formats, −2.81% @ y=11.0 mm and an M58×0.75 thread. 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 — Straight 2/3-inch swap at 35 mm, station measures

PMS-LFEH-3514(A) reproduces the field at 17.9° × 14.3° × 10.8°, focuses from 0.3 m and publishes −0.07% @ y=5.5 mm, small enough that the existing calibration likely survives a verification pass rather than a rebuild. Its 12.82 mm back focal length, M37.5×0.5 thread and 156 g are the items to confirm.

Scenario 2 — Long working distance, camera stands well back

PMS-LFEH-7520M(A) at 75 mm gives 8.4° × 6.7° × 5.0° on 2/3-inch from 0.9 m; PMS-LFEHH-5028M(B) at 50 mm gives 10.08° × 7.57° from 0.3 m at 10 MP. The first suits a fixture that cannot move; the second a station that also needs the resolution class. Neither publishes a back focal length, so check rear clearance physically.

Scenario 3 — 1-inch sensor where resolution is the stated complaint

PMS-LFGH-2516M is the only model here publishing resolving power, so it suits a station whose complaint is softness rather than darkness. It covers 12.8 × 9.6 mm and gives 36.2° × 29.0° × 21.8° on 1-inch, or 25.0° × 20.0° × 15.0° on 2/3-inch — but its 1 m imaging range rules out close fixtures.

Scenario 4 — Large-format wide angle, and the two things that get missed

PMS-LFHHH-1620M covers 4/3-inch from 0.1 m at 16 mm, giving 61.0° × 47.2° with a 73.0° diagonal. The two items that catch people are distortion — −2.81% @ y=11.0 mm, an order of magnitude above the 50 mm models — and the M58×0.75 thread, which no filter from a 2/3-inch station will fit.

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 a specific Computar lens, and none should be inferred. Whether a substitute works depends on the camera's flange distance and front recess, filter glass in the path, the illumination and the algorithm's tolerance.

Computar appears here only as recorded in customer-supplied intelligence: within the POMEAS global company intelligence set of 2,602 companies, 12 have public material indicating Computar C-mount FA lenses in use. No Computar specification is quoted, because none was independently verified.

Optical resolution is not measurement accuracy. Resolving power, where published, is a property of the lens measured on a target; accuracy additionally depends on contrast, edge definition, illumination stability, calibration and the algorithm. A figure such as 160 lp/mm describes what the lens can record, not the smallest dimension a station can hold.

The existing POMEAS article on C-mount lens selection covers the generic method — choosing a lens from scratch, the focal length formula and the full FA series matrix. This page is the narrower case of substituting one C-mount FA lens for another.

FAQ

If the focal length and mount match, is that enough?

No. Image circle, back focal length, closest imaging distance and distortion are independent of those two.

Why does the filter thread matter so much in a lens-to-lens swap?

Because it varies within a line: PMS-LFEH-3514(A) is M37.5×0.5, PMS-LFGH-2516M M33.7×0.5, PMS-LFHHH-1620M M58×0.75. An optically correct swap still fails if the polariser no longer fits.

What if the substitute publishes no back focal length?

Treat it as unknown. PMS-LFEHH-5028M(B) and PMS-LFEH-7520M(A) publish none; check rear clearance physically rather than assuming a sibling's figure.

Is a low distortion figure always better?

Safer, but not free. PMS-LFHHH-1620M's −2.81% @ y=11.0 mm on 4/3-inch is a normal wide-angle figure, manageable with calibration.

How do I compare two lenses quoted at different image heights?

You cannot. A figure at y=5.5 mm and one at y=11.5 mm describe different parts of the field.

What goes into a specification for a discontinued lens?

Sensor format and active area, field of view at the working distance, focal length, maximum distortion with its image height, closest imaging distance, mount, filter thread and temperature.

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.