C-마운트 렌즈란 무엇인가? 플랜지 초점 거리, 수동 조리개 제어, CS-마운트 호환성

C-마운트 렌즈란 무엇인가요?

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. Every C-mount camera body is machined to this FFD, and every C-mount lens is designed to match it, which lets lenses and cameras from different manufacturers work together without optical shimming.

Focus on many C-mount lenses is not a simple slide. An internal cam converts rotation of the focus ring into linear motion of two or more lens groups, moving them relative to each other. This is a compensating system (Kingslake, Lens Design Fundamentals, 2nd ed., §18.3): as the groups shift, their relative motion rebalances off-axis aberrations such as field curvature and astigmatism across the focus range. That is a different mechanism from M12 lenses, which are rigid assemblies focused by threading the whole body in or out.

Minimum object distance (MOD) is a hard mechanical limit: the end of cam travel. Inside MOD the image goes uniformly soft across the field, and an extension tube is needed to focus 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, so a lens focused on the test bench reaches focus on any properly machined C-mount body.

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 can reduce element count and simplify IR-corrected designs at some focal lengths. 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 physically screws into either body. 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 lens sits 5mm too close to the sensor, the image forms behind the sensor plane, and it cannot reach focus at any position. 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 does not work and cannot be corrected. The sensor sits 17.526mm back, 5mm farther than the lens is designed for, and spacers only add path length, never remove it. If you have CS-mount lenses and a C-mount camera, you need different lenses.

CS-mount is common in ITS (intelligent transportation systems) and security cameras, where the shorter flange can simplify IR-corrected optics for 850nm and 940nm license-plate and night work. It is also the native mount on 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.

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 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. Neither is a smaller or larger version of the other. C-mount and M12 are different focus systems.

Cost and integration separate them in practice. C-mount lenses use more elements and tighter cam 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 means a lens and camera from separate vendors reach focus together, while M12 back focal length varies enough that camera-lens pairing should 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.

자주 묻는 질문

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