Cマウントレンズとは？ フランジ焦点距離、手動絞り制御、およびCSマウントとの互換性

Cマウントレンズとは？

A C-mount lens uses a 1"-32 UN thread (one inch outer diameter, 32 threads per inch) and a specified 17.526mm flange focal distance. The thread and flange are the entire mechanical specification. C-mount bodies are machined to that FFD and C-mount lenses are designed against it, which is what makes cross-manufacturer pairing routine. That is nominal geometry, not a guarantee: real parts carry tolerance, and rear-element clearance, focus travel, image circle, and cover-glass thickness stay lens and camera specific.

Focus mechanism belongs to the design, not the mount. Rotating the focus ring on an adjustable C-mount lens may translate the whole optical cell in a helicoid, shift only the front group, or drive internal groups along a cam track; the datasheet for the lens you are evaluating, or a question to Commonlands engineering, settles which, along with how the image behaves at the near limit.

Fixed-focus C-mount products exist too, and all are equally C-mount. That is also different from M12 lenses, which are rigid assemblies focused by threading the whole body in or out.

On an adjustable C-mount lens, minimum object distance (MOD) is a hard mechanical limit: the end of focus travel. Inside MOD the lens cannot be focused, and how the image falls off there depends on the design rather than degrading uniformly across the field; an extension tube focuses closer. MOD is measured from the front of the lens to the object, not from the sensor. Working distance runs from around 100mm to infinity depending on the product. It is not a single fixed range across the catalog.

Beyond thread and flange, the standard says nothing about optical performance or sensor coverage. A 2.8mm wide lens and a 75mm telephoto are both C-mount. The mount is a mechanical interface, not a performance tier.

The 17.526mm figure carries a tight machining tolerance because it sets focus accuracy directly. Commonlands holds flange focal distance to that tolerance. Where a camera's registration or cover glass sits off nominal, expect to refocus or shim.

What Is a CS-Mount Lens?

A CS-mount lens shares the same 1"-32 UN thread as a C-mount lens but uses a shorter 12.526mm flange focal distance instead of 17.526mm. The 5mm shorter optical path lets designers work with shorter back focal lengths, which is design freedom rather than a guaranteed simplification: element count and spectral correction follow from the prescription, field, aperture, and conjugates, not from the flange.

It is not a smaller version of C-mount optically; the flange geometry drives the whole lens design. Because the thread is identical, either lens screws into either body wherever the rear group clears the throat. Whether it focuses is a separate question.

A C-mount lens on a CS-mount camera works with a 5mm C-to-CS spacer adapter that restores the 17.526mm distance. Without it, the flange sits 5mm closer to the sensor than the lens expects: at infinity the image forms behind the sensor plane, and focusing closer moves it further back. No normal focus travel covers a 5mm deficit. Many CS-mount cameras ship with the spacer, and the CLA217-CCS adapter ($6.00) provides the correction.

A CS-mount lens on a C-mount camera loses infinity focus, and no passive spacer brings it back. The sensor sits 17.526mm back, 5mm farther than the lens is designed for, and spacers only add path length, never remove it.

What that extra 5mm does is act as an extension tube, so the pairing still focuses at close range, at working distances far shorter than the lens markings suggest. How close it lands is lens-specific, so measure the pairing rather than assume it. For infinity or normal standoff, use C-mount lenses.

CS-mount is common in ITS (intelligent transportation systems) and security cameras, where the shorter flange suits the wide, fast designs those cameras use. Correction at 850nm or 940nm for license-plate and night work is a property of the design, not the mount.

CS is also the native mount on the C/CS version of the Raspberry Pi High Quality Camera, which takes CS-mount lenses directly and C-mount lenses with the 5mm spacer. For most machine vision, particularly sensors above 2/3" or focal lengths above 16mm, C-mount stays the default. Browse the Commonlands CS-mount collection.

What Is an Industrial Lens?

An industrial lens is a lens selected for machine vision, robotics, automation, inspection, or scientific imaging, where image geometry and repeatability matter more than photographic look. The term is not tied to one mount or price point; it names a set of engineering priorities. Repeatable geometry comes first: once focused and locked, field of view and distortion should stay constant across units and production runs, because a calibrated algorithm cannot tolerate geometry that shifts by serial number.

The table describes what is typical of each category, not what any mount or standard requires; individual products cross the lines.

C-mount and M12 are the two dominant industrial mounts, with CS-mount a close variant of C-mount on a 5mm shorter flange. Neither C-mount nor M12 is universally better. Choose on sensor size, aperture control, and packaging. Commonlands makes lenses in all three mounts, and the C-mount vs M12 comparison below covers the decision.

Sensor Size and Image Circle Coverage

C-mount specifies the mechanical interface, not sensor coverage. Coverage is set by the lens image circle, the diameter of the usable projected image at the focal plane. If the image circle is smaller than the sensor diagonal, the corners go dark, a defect called vignetting. Corners can also go soft where the field exceeds the lens's corrected image circle, a separate resolution effect. Always check image circle against sensor diagonal, or pick a lens rated for a larger format than you need.

Corner performance is not uniform even within a rating. Commonlands publishes an image circle for every C-mount and CS-mount lens, so the check against your sensor diagonal is direct. See the sensor size and lens compatibility guide for format matching.

C-Mount vs M12: Which Should You Choose?

Choose C-mount when you need an adjustable iris for depth-of-field control, sensor coverage above roughly 1/1.8", or focal lengths past 50mm. Choose M12 when size, weight, and cost dominate: embedded cameras, drones, and small robotics platforms are its natural home. C-mount and M12 are different focus systems, not scaled versions of each other.

C-mount lenses use more elements and tighter mechanical tolerances, so unit pricing usually runs above comparable M12 optics, though a well-specified M12 lens for a demanding sensor is not automatically cheaper than an entry C-mount lens. The bigger factor is integration: C-mount's standardized flange gives a lens and camera from separate vendors the same nominal registration, while M12 back focal length varies enough that every pairing must be checked against the holder height in the datasheet.

For the full mount comparison including M8, see the lens mount guide, and browse the Commonlands M12 lens collection for compact options.

よくある質問

Commonlands C-mount lenses pair a manual iris with working distances from typically 100mm to infinity. These answers cover the standard behind them.