Schneider C-Mount FA Lens Alternatives: 6 Parameters to Verify Before Switching

Large-format C-mount installations fail differently from small ones. Where a small-sensor station is limited by whether the lens resolves the pixels it is given, a large-format one is limited first by mechanics: whether the housing has the depth behind the mount for a long back focal length, whether existing filter stock matches this model's thread, and whether the image circle reaches the corners at all.

In the POMEAS global company-intelligence database, 6 of 2,602 surveyed companies have public material recording Schneider optics among their equipment. That figure indicates the deployment class, nothing more: no Schneider specification is quoted below, none having been independently verified.

Six parameters to verify before substituting a Schneider C-mount FA lens

Read them in this order: the first two decide whether the lens is in the right class, the last four whether it fits the station.

1. Focal length and the coverage class you are replacing

Focal length sets the field, so it sets how much of the frame the part fills. For area sensors horizontal field is estimated with the thin-lens approximation:

FOV ≈ sensor width × working distance / focal length

This is an estimate, not a measurement. Using the 17.6 mm width published on the POMEAS 4/3-inch pages, a 400 mm working distance gives roughly 587 mm field with a 12 mm lens, 440 mm with 16 mm and 141 mm with 50 mm.

Establish the class first. The POMEAS line-scan models carry 146 mm over an 82 mm image circle and are matched to line length and magnification instead; comparing that 146 mm with a 12 mm area lens tells you nothing.

2. Image circle versus sensor diagonal: where vignetting starts

Coverage is one comparison: image circle ≥ every corner the sensor uses. Here 4/3-inch is given as 17.6 × 13.2 mm with a φ22 mm circle, and two models also publish 1-inch and 2/3-inch angles, so a smaller sensor can be documented rather than guessed at.

The trap runs both ways. Too small a circle darkens corners and invalidates measurements taken there. Too large a circle is optically safe but usually brings a longer barrel and longer back focal length, and that second-order consequence is what stops installs.

3. Distortion, quoted per format and per field position

Distortion is not one number per lens. It changes with field position, quoted against a field height written as y, and changes with sensor size because a smaller sensor crops out the worst of the field.

This series publishes the point better than most. The 16 mm carries three figures for one field: -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. The 50 mm carries -0.14%, -0.05% and -0.02% across the same three formats. Same family, one order of magnitude apart, and the only honest way to use either set is at the format you actually run.

4. Aperture range and closest focusing distance

Every area model here shares one aperture range, F2.0–F22, which describes light and depth of field and nothing about suitability.

The companion value is not published equally. Only the 16 mm publishes a focus range, 0.1 m to ∞; the 12 mm and 50 mm publish no closest distance. That gap bites hardest on the 12 mm, because an 89° lens is exactly the one people try to use close to the part.

5. Back focal length in air

Distance from the rear reference surface to the image plane, measured in air, and the number that decides whether focus is reachable even when the thread fits. If the flange geometry of the camera and the back focal length of the lens disagree, focus lands everywhere except where it is needed.

Large-format lenses carry long values: 18.93 mm, 21 mm and 20.29 mm here. The 16 mm page lists its value without the "in air" qualifier its siblings carry, so state the condition when comparing. The line-scan model publishes nothing.

6. Mount, filter thread and operating temperature

The mount is C-mount for the area lenses and M72 for line scan, and nothing carries across that boundary. Within the area series the filter thread changes three times: M77 × 0.75 on the 12 mm, M58 × 0.75 on the 16 mm and M40.5 × 0.5 on the 50 mm. Filters bought for one member will not fit another.

Temperature is published at -10 °C to +50 °C across all four models. That consistency is useful; it is also the only environmental value any page publishes.

Where POMEAS optics land on those six

Values published on the POMEAS product pages for three 4/3-inch C-mount area lenses and one M72 line-scan lens:

Reading the table against the six checks:

- 12 mm. Widest at 89° × 75.5° × 61.1° on 4/3-inch, longest published back focal length at 18.93 mm in air, filter thread M77 × 0.75. No focus range published. - 16 mm. The only area model publishing a focus range, 0.1 m to ∞, and distortion across three formats; 61.0° × 47.2° on 4/3-inch, 44.2° × 33.6° on 1-inch, 31.0° × 23.4° on 2/3-inch. - 50 mm. Lowest distortion here by an order of magnitude, to -0.02% @ y = 5.5 mm, with 20.29 mm back focal length. - Line scan. The LLC1 publishes mount, aperture, image circle, distortion below 0.002% and -10 °C to +50 °C — but no working distance.

Specifications published on different product pages were not necessarily measured under identical conditions and should not be read as a ranking.

What those numbers mean on the line

Scenario 1 — Large part that must be framed in one shot

Coverage dominates, and the 12 mm at 89° diagonal is the candidate. Take it knowing two unpublished things: no focus range, so how close it will go is unknown until asked, and -2.4% @ y = 11.0 mm distortion on 4/3-inch, the price of that field.

Scenario 2 — The output of the station is a dimension

Then distortion is the first specification, not the last. Across the same family at the same 4/3-inch field height the published figures run from -2.81% to -0.14% @ y = 11.0 mm. Nothing about the thread, the mount or the pixel count changes that; only focal length does, and only together with whatever compensation the algorithm applies afterwards.

Scenario 3 — Line-scan station losing its lens with no working distance on record

The LLC1 publishes an 82 mm image circle, M72 mount, F4.0–F16 and distortion below 0.002%, but no working distance. Its sibling LLC2 publishes 179–690 mm with three magnification pairs: 1.75× at 179 mm, 2.0× at 632 mm, 2.25× at 690 mm. Without your magnification or standoff, neither page becomes a choice.

Scenario 4 — Mechanics before optics in a tight housing

Three realities arrive together: image circle of φ22 mm, back focal length up to 21 mm, and a filter thread that changes across the series. Check the depth behind the mount and the diameter of the barrel before ordering, and treat any existing M77 or M58 accessory stock as model-specific rather than family-wide.

Writing a replacement specification when the original lens is discontinued

Once the original part is gone, the specification you write is what separates you from guessing twice. Write it in checklist order, leaving unavailable values as not published rather than importing them from another model.

Start with what the station fixes: sensor format and active area, or line length where the camera scans; pixel pitch; and the working distance the mechanics allow.

Then require the lens values with conditions: image circle in millimetres beside the format name; focal length; field angles or line coverage for every format run; distortion per format and field position; aperture; focus range or closest distance; back focal length in air, condition stated; mount; filter thread; temperature. Request anything absent.

Close by saying how acceptance will be judged. Dimensional stations are signed off against known parts, not against images that look sharp, so name the feature sizes and tolerances the replacement has to recover.

What this comparison does not tell you

This is a parameter reference, not a compatibility statement. No claim is made that any POMEAS lens is a drop-in replacement for another manufacturer's lens, and none should be inferred. Whether a substitute works depends on the mechanical envelope, the flange geometry of the camera, whatever sits between mount and sensor, and the illumination.

The Schneider name appears only where documented in customer-supplied intelligence: evidence of what the market has installed, not a source of third-party specifications, and no Schneider figure is reproduced here.

Optical performance is not measurement accuracy. Distortion and field angles describe the lens; what the station reports also depends on pixel sampling, contrast, illumination stability, encoder synchronisation on line-scan stations, and the algorithm.

This page covers 4/3-inch C-mount area lenses and one M72 line-scan lens. It does not cover other sensor sizes, nor zoom behaviour across a range.

FAQ

Why is back focal length larger on a 4/3-inch lens?

Larger image circles generally come with longer back focal lengths. The models here publish 18.93 mm, 21 mm and 20.29 mm, so the depth behind the mount has to be checked.

Does one distortion figure describe the lens across all sensors?

The 16 mm publishes -2.81% at y = 11.0 mm on 4/3-inch, -1.92% at y = 8.0 mm on 1-inch and -1.06% at y = 5.5 mm on 2/3-inch: three figures for one lens, and the applicable one depends on the sensor.

Do filters and hoods transfer within one lens series?

Threads differ per model: M77 × 0.75 on the 12 mm, M58 × 0.75 on the 16 mm and M40.5 × 0.5 on the 50 mm — all C-mount, all one family.

Which of these can focus close to the part?

Only the 16 mm publishes a focus range, 0.1 m to ∞. The 12 mm and 50 mm publish no closest distance, so nothing should be assumed about them until the value is requested.

What can be done without knowing the line-scan working distance?

The LLC1 publishes no working distance, image circle aside; LLC2 publishes 179–690 mm with three magnification pairs. Either your magnification or your standoff must be known to choose.

Is the widest lens usable for measurement?

It frames large parts, at 89° diagonal and -2.4% distortion at y = 11.0 mm on 4/3-inch. Presence and location work well; dimensional output needs calibration against the specific unit rather than the family average.

Talk to the engineering team

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.

https://www.pomeas-vision.com/Contact.html

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