What Is a C-Mount Lens? Flange Focal Distance, Manual Iris Control, and CS-Mount Compatibility
This guide covers the 1"-32 UN thread, the 17.526mm flange focal distance, cam-driven focus, the adjustable iris, CS-mount compatibility, and when C-mount beats M12.
A C-mount lens is a screw-thread lens standard using a 1"-32 UN thread and a fixed 17.526mm flange focal distance (FFD), the distance from the mount flange to the sensor plane. Many C-mount lenses focus through an internal cam mechanism that moves optical groups relative to each other, rebalancing aberrations across the focus range, and include a manual iris ring for aperture control.
C-mount is the default industrial lens mount for machine vision: broad focal length selection, sensor coverage up to 1.2" and beyond on select lenses, and an adjustable iris that most M12 lenses do not have. CS-mount shares the thread with a shorter 12.526mm flange, and M12 is a different, smaller system entirely.
What Is a C-Mount Lens?
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.
| Parameter | CS-mount | C-mount |
|---|---|---|
| Thread | 1"-32 UN | 1"-32 UN |
| Flange focal distance | 12.526mm | 17.526mm |
| Typical max sensor coverage | ~2/3" | 1.2" and larger on select lenses |
| Typical focal length range | ~2.5mm-16mm | 2.8mm-100mm+ |
| Common use | CCTV, ITS, security, Raspberry Pi HQ prototyping | Industrial machine vision, scientific imaging |
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 a Manual Iris Lens?
A manual iris lens has a physical iris ring the user rotates to set the aperture (f-number), then locks. There is no motor and no feedback circuit. That differs from a fixed-iris lens, common on M12, where aperture is set at the factory. It also differs from an auto-iris lens, which uses an electronic actuator to track scene brightness. Manual iris is the standard on C-mount machine vision lenses.
Adjustable iris control is the single biggest practical advantage of C-mount over M12 for machine vision. Stopping down raises depth of field; opening the iris lowers it. Because machine vision illumination is usually LED-based and programmable, engineers can stop down to gain depth of field and then drive the lighting harder to recover the lost throughput. That tradeoff is impractical in ambient-light photography but routine on a production line. Commonlands C-mount lenses carry a manual iris for exactly this control.
| Iris type | Who sets aperture | Typical use |
|---|---|---|
| Manual iris | User, with a ring, locked before deployment | C-mount machine vision with programmable lighting |
| Fixed iris | Factory-set, not adjustable | M12 embedded and low-cost applications |
| Auto iris | Electronic actuator responding to scene brightness | Outdoor CCTV, variable-lighting surveillance |
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.
| Property | Industrial lens | Consumer lens | CCTV lens |
|---|---|---|---|
| Distortion specification | Published and characterized | Often not specified for machine vision | Rarely specified, typically higher |
| Aperture control | Manual iris (C-mount) or fixed (M12) | Electronic, via camera body | Manual iris, DC auto-iris, or fixed |
| Focus locking | Set screw or locking ring | Not applicable (autofocus) | Sometimes, often friction-only |
| Mount standard | C-mount, CS-mount, M12 | Proprietary (E, F, RF, L) | C-mount, CS-mount |
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.
| Sensor format | Approx. diagonal | Typical lens tier |
|---|---|---|
| 1/3" | 6.0mm | Entry C-mount, CS-mount, CCTV |
| 1/2" | 8.0mm | Mid-range M12 and C-mount |
| 2/3" | ~11.0mm | Standard industrial C-mount |
| 1" | ~16.0mm | Higher-resolution C-mount |
| 1.1" | ~17.6mm | High-resolution 12-25MP industrial |
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.
| Parameter | C-mount | M12 / S-mount |
|---|---|---|
| Thread specification | 1"-32 UN | M12 x 0.5 |
| Flange focal distance | Fixed at 17.526mm | No standard, varies by lens |
| Focus mechanism | Internal cam moves groups, rebalancing aberrations across focus range | Rigid body threaded in/out, no internal rebalancing |
| Iris control | Adjustable iris ring on many lenses | Typically fixed aperture, no iris ring |
| Working distance | Varies by product, typically 100mm-infinity | 50mm-infinity (uncorrected) |
| Typical sensor coverage | Up to 1.2" and larger on select lenses | Most models up to 1/1.8" (select designs 1/1.7"-1/1.6") |
| Focal length range | 2.8mm-100mm+ | 0.8mm-100mm |
| Typical lens mass | Heavier | Lighter |
| Best for | Industrial inspection, larger sensors, DOF control | Embedded, drones, robotics, compact sensors |
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.
Top 5 C-Mount and CS-Mount Lenses for Machine Vision
The five Commonlands lenses below cover common C-mount machine vision roles, from an 8mm wide-area lens (CIL531, 2/3", F/2.8) to a 25mm high-resolution option for 1.1" 20MP sensors (CIL544, F/1.8). Four are C-mount. The CIL725 2.5mm fisheye is CS-mount, included for Raspberry Pi HQ and panoramic builds. Every lens carries an adjustable iris, the control that separates C-mount from most M12 optics.
| Rank | Lens | Mount | EFL | F-number / iris | Format coverage | Best for |
|---|---|---|---|---|---|---|
| 1 | CIL531 8mm | C-mount | 8mm | F/2.8, adjustable | Up to 2/3", 12MP | Wide-area overhead robotics and large-area inspection |
| 2 | CIL522 12mm | C-mount | 12mm | F/1.4, adjustable | Up to 2/3", 3MP at 4.2 micron | Bright general-purpose inspection baseline |
| 3 | CIL535 35mm | C-mount | 35mm | F/2.0-F/16, adjustable | Up to 2/3", 12MP at 2.2 micron | Longer standoff conveyor and gantry inspection |
| 4 | CIL544 25mm | C-mount | 25mm | F/1.8, adjustable | 1.1", 20MP+ at 2.74 micron | High-resolution measurement on large sensors |
| 5 | CIL725 2.5mm | CS-mount | 2.5mm | F/1.8-F/14, adjustable | Up to 2/3", 12MP (6.4mm fisheye image circle) | Panoramic and Raspberry Pi HQ builds |
How we picked: every lens is stocked at Commonlands with a published image circle and an adjustable iris, and each fills a distinct role so the five span wide-area through high-resolution work. Specs match the product datasheets. Size focal length from your working distance and sensor width in the EFL calculator, and confirm image circle against your sensor diagonal before ordering.
Fujinon, Kowa, and Computar are the established C-mount FA lens makers, and Edmund Optics stocks a broad C-mount catalog under its TECHSPEC line. Their ranges run deeper at long focal lengths and formats above 1.1", so they remain the reference point for those cases. Commonlands competes on direct pricing, same-day San Diego stock, and direct access to the engineers who specify the optics.
Frequently Asked Questions
Commonlands C-mount lenses pair a manual iris with working distances from typically 100mm to infinity. These answers cover the standard behind them.
What is a C-mount lens?
A C-mount lens is a screw-thread lens using 1"-32 UN thread and a 17.526mm flange focal distance. The standard defines the mechanical interface between lens and camera body, so lenses and cameras from different manufacturers work together. Many C-mount lenses focus through an internal cam that rebalances aberrations across the focus range and include an adjustable iris ring.
What is the flange focal distance of a C-mount lens?
17.526mm, measured from the lens mount flange to the sensor plane. C-mount camera bodies are machined to this distance, and C-mount lenses are designed with a matching flange focal distance. CS-mount uses the same thread with a 12.526mm flange instead, a 5mm difference that governs adapter use.
Can a C-mount lens be used on a CS-mount camera?
Yes, with a 5mm C-to-CS spacer adapter that restores the 17.526mm flange distance. Without it, the lens sits 5mm too close to the sensor (the image forms behind the sensor plane) and cannot reach focus at any position. Many CS-mount industrial cameras ship with this spacer included. The CLA217-CCS adapter provides the correction for $6.00.
Can a CS-mount lens be used on a C-mount camera?
No. A CS-mount lens is designed for a 12.526mm flange distance, and a C-mount camera places the sensor 17.526mm back. Spacers only add optical path length. None can shorten it. This compatibility rule is one-directional, and there is no adapter fix.
When should I choose C-mount over M12?
Choose C-mount when you need an adjustable iris for depth-of-field control, sensor coverage above roughly 1/1.8", or a wide range of focal lengths including options past 50mm. Choose M12 when size, weight, and cost are the dominant constraints, such as embedded cameras, drones, and small robotics platforms. See the lens mount guide for a detailed breakdown.
Need Help Choosing a C-Mount Lens?
Commonlands manufactures C-mount, CS-mount, and M12 lenses for machine vision and embedded vision. Lens designs are MTF characterized against design specifications. Send our San Diego engineering team your sensor model and application at engineering@commonlands.com or call +1 (858) 333-7325. Orders placed before 12 PM PST ship same day.