C-mount lens on an M16 board camera: adapters and flange distance

A board camera with an M16 lens holder takes short, light board lenses, and nothing else. The day the task calls for a narrow field of view at distance, you want C-mount glass: 25 mm and 50 mm lenses are common, sharp and easy to find. The M16-to-C adapter you are searching for, though, does not exist, and for a physical reason rather than a missing product listing. Below are the flange-distance numbers behind that, then the conversion that works: swap the holder for a CS-mount board holder on the standard 20 mm fixing-hole spacing, add a 5 mm ring for C lenses, and shim to a measured focus. A worked example sizes the whole decision for a 25 mm lens on a 1/2.9-inch sensor at 10 m.

Why an M16-to-C adapter does not exist

C-mount is a precision datum, not only a thread. The standard sets the lens flange face 17.526 mm from the image plane, on a 1 inch, 32 threads-per-inch mount. CS-mount keeps the same thread at 12.5 mm, and the 5.026 mm between the two is exactly why a 5 mm ring lets a CS camera accept C lenses. This is the same flange mechanics covered in camera lens adapter focus: fit and flange distance and, for the C and CS pair itself, C-mount vs CS-mount adapters.

An M16 holder has no datum at all. It is a threaded bush screwed into a plastic tower to whatever depth the module designer chose, so the lens-to-sensor distance differs board to board and changes each time a lens is screwed in and out. A ring that adapts M16 to the C thread would carry the lens but still leave nothing that sets 17.526 mm: every copy needs hand-setting, and any knock moves it. The heavy C barrel also loads screws and plastic sized for gram-scale board lenses. The missing part is not an adapter; it is a reference face.

Three routes to C-mount glass, and where each ends

A middle option, a C/CS switchable holder, carries the 5 mm offset on a locking collar so you can trim focus without washers. It suits a bench or a prototype, but it is the wrong choice for a machine that must hold calibration — see the fixed-holder trade-off in the FAQ below. If the board is staying, convert the holder; if the camera is being chosen anyway, start from one with the mount already right — the board level camera selection guide sorts the rest of that decision.

What you need

• A metal CS-mount holder, or a C/CS switchable holder, with 20 mm between its fixing holes. Holders are made to the 20 mm board standard, but measure the board with calipers before ordering: holders exist at other spacings, and a millimeter of error means a dead part.
• A 5 mm CS-to-C adapter ring, if the lens is C-mount.
• Depth calipers or a depth gauge; 0.1 mm resolution is enough.
• Shim washers or shim stock in 0.1 mm, 0.2 mm and 0.5 mm thicknesses.
• An IR-cut filter if the sensor is color. Many holders include one in the mount face; near-infrared contamination and IR-cut filter options explains what it does.
• A high-contrast target at the working distance, medium-strength thread locker, and the driver the board screws take.

Swapping the holder, step by step

The swap is screws and arithmetic; nothing on the board is soldered.

1. Measure the fixing-hole spacing and screw size on the board with calipers. Holders are made to 20 mm spacing; confirm yours before ordering.
2. Survey the board around the optics. The C and CS thread is 1 inch across, so the new mount and lens cover more area than the M16 bush did, and a long lens reaches down toward the board. Move connectors, tall parts and cables out of that footprint first.
3. Remove the two small self-tapping screws holding the M16 holder and lift the holder off.
4. Seat the new CS holder on its set screws. Keep its IR-cut window for a color sensor; a mono sensor can go without.
5. Depth-gauge from the mount face down to the sensor cover glass and write the reading down. The CS standard is 12.5 mm to the image plane; a glass reading runs a few tenths low, and that offset is constant, so it cancels when you correct by differences.
6. Fit the 5 mm ring for a C-mount lens. Without it the lens sits 5.026 mm too close and can only focus close in, the classic focuses only at close distance fault.
7. Fit the lens, aim at the working-distance target and set focus with the aperture wide open, where depth of field is thinnest and errors show.
8. If focus runs out before the target is sharp, shim in 0.1 mm steps: washers between holder and board raise the whole mount, while a thin washer under the lens shoulder moves only the lens. Re-check after each step.
9. Thread-lock the set screws, let the locker cure, then re-check focus across the corners of the field. The corner check in M12 lens focusing: adjust, check corners and lock applies unchanged.

Set and verify the flange distance

Two numbers rule the result: 12.5 mm for CS glass and 17.526 mm for C glass, both to the image plane. A stack that misses them shows up as focus range, not as blur everywhere at once. Short stack: the lens cannot reach far focus, so mid-range looks fine and distant targets drift soft. Tall stack: close focus recedes until the working distance itself is out of reach.

Shim to the deficit, not to a feeling: measure, subtract, add the difference, as the worked example below carries through in full. The close-focus side of a tall stack is the thread-travel trade described in M12 lens thread travel, and the spacing-to-focus relationship itself is worked through in lens focal length vs back focus in board cameras.

Verify on a target at the longest distance the task uses, wide open, and stop down only after the far target snaps; stopping down early hides a half-shim error behind depth of field. Re-check after the first warm-up, because board cameras drift with temperature, and focus drift with temperature shows how to separate drift from a shimming fault. A printed target makes the check repeatable between shifts. The LS-LXR-0029 is a 76 x 76 mm dot-grid calibration target with 0.5 mm dots.

Worked example: a 25 mm C-mount lens on a 1/2.9-inch board camera at 10 m

Inputs, with their sources:

• Focal length 25 mm — the C-mount lens fitted after the conversion.
• Sensor 1/2.9-inch, 1080p class: 1920 × 1080 pixels at 2.9 μm, so 1920 × 2.9 μm = 5.6 mm wide.
• Working distance 10 m — the long-distance reading task the conversion is for.
• Standards: 12.5 mm CS flange, 17.526 mm C flange, 5.026 mm of ring between them.
• Measurement: the converted holder depth-gauges to 12.2 mm, the few-tenths-short reading these conversions typically leave.

The rules, in words: a lens fills a field that grows in proportion to distance, so field width equals working distance times sensor width divided by focal length. The stack must put the mount face 12.5 mm from the image plane for CS, or 17.526 mm for C once the ring is on, and the shim closes whatever the calipers say is missing.

Field width: 10,000 mm × 5.6 mm ÷ 25 mm = 2,240 mm, about 2.2 m across at 10 m. The same sensor at 8 mm reaches 7,000 mm ≈ 7 m, at 12 mm about 4.7 m, and at 50 mm about 1.1 m. Stack: 12.2 mm measured plus the 5.026 mm ring gives 17.226 mm against 17.526 mm required, so the stack is 0.3 mm short; a 0.2 mm and a 0.1 mm washer close it. Left unshimmed, that 0.3 mm at a 25 mm focal length lands true infinity near f² ÷ 0.3 mm = 625 ÷ 0.3 ≈ 2,080 mm, about 2 m, so the lens snaps a target at 2 m while anything farther drifts soft.

Decision, with its threshold: the conversion earns its hour when the task needs a field narrower than about 3 m at 10 m. At 25 mm the field is 2.2 m, so convert or move to a long M12 lens. If 7 m of coverage is acceptable, the original holder with an 8 mm lens does the job untouched. Between the two, a 12 mm lens at about 4.7 m usually ends the discussion before a screwdriver comes out. The focal-length logic behind these numbers is laid out in choosing a machine vision lens focal length, and same lens on a different sensor shows why the sensor-width term moves the answer so much.

The M12 route reaches these focal lengths directly. The LS-MBL-0031 is a 25 mm M12 board lens made for a 1/1.8-inch sensor at 5 MP. On the 1/2.9-inch sensor of the example it covers the same frame with room to spare, with no screws turned. Check the M12 board lens shelf before spending the retrofit hour.

Common mistakes

• Ordering by eye. The 20 mm spacing is the common one, not the only one; calipers before checkout.
• Forgetting the 5 mm ring. A C lens without it sits 5.026 mm too close and behaves as a macro. Extra spacing is legitimate when close-up is the goal — extension tubes for close-up machine vision — but a 15 mm tube is a macro tool, not a 0.3 mm correction.
• Plastic or printed rings. A printed adapter bends under lens weight and moves with temperature; metal only.
• Color sensor without IR cut. Near-infrared leaks into every channel and flattens contrast; mono sensors skip the filter.
• One shim and done. Shim in 0.1 mm steps and re-check; a corner left soft usually means the holder sits tilted, so re-seat rather than stack more.

Which route to take

Existing board, committed C-mount lens: swap the holder, fit the ring, shim to the calipers. Under an hour, fully reversible.

New build: start from a camera with the mount settled at the factory. The LS-BCM-0020 is a 6.3 MP board camera on the IMX178 sensor with a CS-mount already fitted and a GigE interface.

Narrow field of view only: check long focal lengths in the M12 board lenses first; the retrofit earns its place when the M12 shelf stops short or a specific C-mount lens is the requirement. The M12 lens selection guide covers format, focal length and IR cut before you order.

The lens finder filters M12, CS and C-mount optics by focal length, the lens configurator pairs a lens with the sensor, and the board camera finder lists cameras whose mounts are already settled. For a retrofit that fights back, send board photos and caliper numbers through the contact page; the rest of the mount-and-focus articles sit on the lenses topic page.