Replacing a C-Mount Lens on an Existing Vision Cell: 6 Specs to Verify

Why lens substitution is a specification problem, not a part-number problem

Across 2,602 machine vision companies in the POMEAS global intelligence database, Cognex appears in 119 company profiles, Basler in 42, Hikrobot in 32, Omron in 12, Computar in 12, Kowa in 10 and Schneider in 6. These are the optical and camera brands most frequently named in public descriptions of existing inspection equipment.

That frequency tells you something useful and something limited. It tells you a C-mount industrial lens is the most common optical interface in installed vision cells. It does not tell you that any specific lens can be swapped for another.

A lens substitution fails for ordinary, checkable reasons: the image circle does not cover the sensor, the resolution class does not match the pixel count, the flange focal distance is wrong, or the mechanical barrel fouls the camera front. Every one of those is a published number. This article lists the six numbers to read before ordering a replacement, and shows how POMEAS FA lenses publish them.

It is a verification checklist, not a cross-reference table. Nothing here states or implies that a POMEAS lens is a drop-in replacement for any third-party model.

1. Sensor coverage: the number that causes dark corners

The image circle of the lens must be at least as large as the diagonal of the camera sensor. If it is smaller, the corners go dark.

POMEAS FA lenses publish the intended sensor format in the model name and title. The line is organised in three coverage tiers:

A 2/3-inch lens on a 1-inch sensor will vignette. A 1-inch lens on a 2/3-inch sensor will not — the image circle is larger than needed and the corners stay covered.

What to verify: the sensor diagonal of your actual camera, not the sensor format printed on the housing. A 1-inch sensor named "1 inch" has a diagonal well under 25.4 mm; the naming is historical, not literal.

2. Resolution class: match it to the pixel count, not to the megapixel marketing

A lens rated for 5 MP will not resolve the detail a 12 MP sensor can capture. The result is not a failure — it is a soft image where the extra pixels record nothing new.

The POMEAS FA line publishes three resolution tiers:

- 5 MP class — PMS-LFEH, PMS-LFGH - 10 MP class — PMS-LFEHH (2/3 inch), PMS-LFHHH (4/3 inch) - 12 MP class — PMS-LFHHG (4/3 inch)

What to verify: the pixel count of the camera and the size of the smallest feature you must resolve. If the feature is 20 µm on a 4/3-inch sensor, compute the required magnification first, then check whether the lens resolution class supports it at that magnification.

3. Focal length: derive it from the field you actually need

The relationship is:

` f = (sensor width × working distance) / field of view width `

All three terms in millimetres. Worked example: a 1-inch sensor roughly 12.8 mm wide, a working distance of 300 mm, and a required horizontal field of 100 mm:

` f = (12.8 × 300) / 100 = 38.4 mm `

The nearest published options in the POMEAS 1-inch line are 35 mm. The 4/3-inch 10 MP line also carries 35 mm. If neither the focal length nor the working distance can move, the field of view is fixed — you change the camera, not the lens.

What to verify: whether the focal length you computed exists as a catalogue item. FA lenses come in discrete focal lengths; a computed value of 38.4 mm is not orderable and you must move to 35 mm and accept a slightly wider field, or adjust the working distance.

4. Working distance and the mechanical envelope

Two separate constraints hide here.

Optical: the lens has a minimum object distance. Closer than that, it will not focus. The specified working distance range must include the distance your fixture actually provides.

Mechanical: the barrel diameter and length must clear the camera body, any filter thread, and the illumination. A longer focal length usually means a longer barrel. On a crowded station, a 75 mm lens may not physically fit where a 25 mm one did.

What to verify: measure from the camera flange to the part surface, and compare it against the lens specification rather than against the outgoing lens.

5. Distortion: only critical when the output is a dimension

Distortion matters when the station measures. It does not matter much when the station only judges presence or absence.

- For pass/fail presence inspection, a few percent of barrel distortion is usually tolerable; the feature is either there or it is not. - For dimensional output, distortion enters the measurement directly. A 2% distortion at the edge of a 100 mm field is a 2 mm error — far larger than most tolerances.

Several models in the POMEAS FA line are published as low-distortion variants (the "低畸变" designation appears on the 12, 16 and 35 mm PMS-LFEH and the 35 mm PMS-LFGH and PMS-LFHHG).

What to verify: the stated distortion figure and the conditions under which it was measured. Distortion numbers from different manufacturers are not comparable unless the measurement method, the sensor format and the field position are all stated. If the method is not published, treat the number as unusable for a dimensional station.

6. Mount and flange focal distance

C-mount specifies a 17.526 mm flange focal distance and a 1-inch 32 TPI thread. CS-mount specifies 12.526 mm. The two are mechanically interchangeable only with a 5 mm spacer, and only in one direction.

What to verify: which mount the camera has, and whether a spacer is present. Fitting a C-mount lens to a CS-mount camera without the spacer will not reach focus at infinity.

Where POMEAS FA optics sit on these six

Real application scenarios

A packaging line changing camera vendor. The cell was built around a 2/3-inch camera and a 25 mm lens. The new camera has a 1-inch sensor. The existing 25 mm 2/3-inch lens will vignette. The check is sensor coverage first, focal length second — a 25 mm 1-inch lens (PMS-LFGH-2516M) keeps the field of view while covering the larger sensor.

A PCB station that needs finer defect detection. The station inspects at 5 MP and needs to resolve a smaller feature. Moving to a 12 MP camera without changing the lens gains nothing: the 5 MP lens is the limiting element. The check is resolution class against the pixel count.

A dimensional station that keeps drifting. The lens is a general-purpose model on a station that outputs measurements. The check is the distortion figure and its measurement conditions, not the megapixel rating.

What this checklist does not tell you

- It does not tell you whether any specific POMEAS lens replaces any specific third-party lens. That determination requires both datasheets and, for a dimensional station, a measurement on the actual part. - It does not cover filter threads, iris control (manual vs. motorised), or temperature stability. Those are per-model and belong on the product page. - Third-party specifications are not reproduced here. Nothing in this article states a specification for any brand other than POMEAS.

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FAQ

Can I substitute a different brand of C-mount lens on my existing camera?

Mechanically, yes if both are C-mount. Whether the station still works depends on four published numbers: sensor coverage, resolution class, focal length and flange focal distance. Compare those before ordering, not the brand name.

What is the difference between C-mount and CS-mount?

C-mount specifies a 17.526 mm flange focal distance; CS-mount specifies 12.526 mm. The 5 mm difference is bridged by a spacer. Fitting a C-mount lens to a CS-mount camera without the spacer will not reach infinity focus.

Why does the replacement lens vignette when the old one did not?

The image circle does not cover the sensor. This usually happens when the camera is changed to a larger sensor format and the lens is kept. Check the sensor format the lens is built for against the sensor actually fitted.

Do I have to change the lens when I change the camera?

Only if the sensor format changes. Moving from a 2/3-inch to a 1-inch sensor requires a lens built for 1 inch; keeping the old lens produces dark corners. A change of pixel count at the same sensor size requires checking the resolution class instead.

My computed focal length is not a catalogue item. What now?

FA lenses come in discrete steps. Move the working distance so a catalogue focal length lands on your required field, or accept the nearest step and recompute the field of view it gives.

Is low distortion necessary for every station?

Only where the output is a dimension. For pass/fail presence inspection, distortion usually does not affect the result. Where it does matter, verify the figure and the conditions it was measured under.

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Next Step

Send the camera sensor format, pixel count, working distance and required field of view. POMEAS will return the FA lens focal lengths that cover it, with the calculated field of view for each option.