C-mount lens thread and diameter: the 1 inch-32 UNS standard

A spec sheet reads C-mount and stops there. The useful numbers are fixed by a standard written for 16 mm cine cameras a century ago and carried unchanged into machine vision: a 1 inch-32 UNS thread, a 25.4 mm nominal barrel diameter and a 17.526 mm flange focal distance. This guide gives you those numbers with the arithmetic behind them, explains why any C-mount lens threads onto any C-mount camera, and separates the checks the mount settles for you from the ones it leaves to you: sensor coverage, back focus and the CS-mount adapter.

What thread does a C-mount lens use?

A C-mount lens carries a male 1 inch-32 UNS thread — a nominal 1 inch (25.4 mm) diameter with 32 threads per inch, or 0.794 mm between thread crests — and it mates with the matching female thread cut into the camera's front plate.

The designation reads like any unified thread: 1 inch of major diameter, 32 threads per inch, and UNS because 32 TPI is a special pitch outside the regular coarse and fine 1-inch series. The lens holds the male (external) thread, the camera the female (internal) thread. They engage over a few millimetres while the lens's rear flange face seats against the camera's front plate; that machined seat, not thread friction, positions the optics. The dimensions above follow the published summary in the C-mount entry.

Keep 0.794 mm in mind for focusing. Each full turn of the barrel moves the lens 0.794 mm along the optical axis, and on a fixed C-mount lens that is the finest focus adjustment you have — which is why small flange errors are corrected in fractions of a turn.

What diameter is a C-mount lens barrel?

The nominal barrel and thread major diameter is 25.4 mm — exactly 1 inch — so a C-mount lens needs a clearance bore of about 26 mm in any panel or bracket it passes through.

That clearance is the 25.4 mm thread diameter plus 0.6 mm of slack: 25.4 mm + 0.6 mm ≈ 26 mm. The standard controls only the threaded section at the mounting face; behind it, focus barrels, iris rings and connectors can be wider, and overall length varies by design. The camera side mirrors the lens: a female 1-32 UNS bore whose major, minor and pitch diameters follow the standard internal-thread tables, as tabulated for a different size in 3/4-40 UNS internal thread specs: major, minor and pitch diameters. When you machine a mounting plate, cut the plain clearance hole at 26 mm and drill any threaded hole to the internal 1-32 UNS dimensions.

How far from the sensor does a C-mount lens sit?

The standard fixes the flange focal distance at 17.526 mm — the distance from the lens's mounting face to the sensor — and every C-mount camera holds its sensor within a fraction of a millimetre of that plane.

Lens designers place infinity focus against that plane, so the flange distance is why distance focus lands where it should. Cameras meet it with machined tolerances; when a unit drifts, the correction is mechanical, and C-mount flange focal distance tolerance: fix focus with shims walks the shimming procedure. In imperial units the same figure is 17.526 ÷ 25.4 = 0.690 inch, the value you will see on drawings written in inches. Everything below refers to this number simply as "the flange distance".

What changes when the camera is CS-mount?

A CS-mount camera puts the sensor 12.5 mm from the flange — exactly 5 mm closer than C-mount — so a C-mount lens seated directly on a CS camera cannot reach focus at distance.

The thread itself is identical: a CS camera takes the same 1-32 UNS lens. Only the sensor position differs, and the adapter is the whole fix. The arithmetic: 17.526 mm − 12.5 mm = 5.026 mm, which the industry sells as a nominal 5 mm C-to-CS spacer ring. Without it, the lens sits 5 mm farther from the sensor than its optics expect, so distant targets stay soft no matter how you turn the barrel, while close targets can still be sharpened. Mount choice, clearance and the adapter trade-offs are laid out in C-mount vs CS-mount: adapters, focus and clearance.

Why does any C-mount lens fit any C-mount camera?

Because the standard fixes the three things that must match — a 1-32 UNS thread, the flange distance and a machined seating face — thread fit and focus range are guaranteed by the mount itself.

The interchangeability was the point. The mount was written for 16 mm cine cameras, where one maker's lens had to mount on another maker's body, and machine vision kept it unchanged. A 2/3-inch industrial lens, a 1.1-inch high-resolution lens and a 0.5x macro lens all carry the same male thread and expect the same plane; only the optics behind the flange differ. Same mount, different glass:

Values as printed in each product's data sheet; open a part for its drawing and full specifications.

Does a C-mount lens cover every sensor?

No — the mount guarantees only mechanical fit; a C-mount lens covers your sensor only if its printed image circle exceeds the sensor diagonal, which is about 11 mm for a 2/3-inch sensor, 16 mm for 1-inch and about 17.6 mm for 1.1-inch.

Worked example: a 35 mm lens on a 2/3-inch sensor

A 2/3-inch sensor's active area is 8.8 × 6.6 mm. The diagonal is √(8.8² + 6.6²) = √(77.44 + 43.56) = √121 = 11.0 mm, so the lens must print an image circle of at least 11 mm. Horizontal field of view at a 35 mm focal length is 2 × atan(8.8 ÷ 70) = 2 × 7.17° ≈ 14.3°. The LS-FFL-0679 prints a 35 mm focal length, a 2/3-inch sensor format, 6 MP resolution and an 8-11 mm image circle:

Larger formats raise the bar. Sensor naming works out at roughly 16 mm of diagonal per inch of type, so a 1.1-inch sensor needs about 17.6 mm of image circle. The LS-FFL-0567 is a 16 mm, 1.1-inch, 20 MP C-mount lens with an F2.8-22 iris, under 0.3% distortion and a 16-22 mm image circle:

Run your own sensor and focal length through the lens focal length calculator:

Format, focal length and the field they produce are treated fully in Lens field of view: why sensor size and focal length set it.

What should you check before threading a lens on?

Check four things in order — clean undamaged threads, the correct mount (C or CS), image circle against sensor diagonal and back focus at distance — before you lock the barrel.

1. Threads. A 32 TPI thread is fine-pitched and strips if forced. Start it straight by hand; if it binds, back off and inspect rather than driving it — the full diagnosis is in Camera lens mount thread binding: safe checks before fitting.
2. Mount type. Confirm whether the camera is C or CS. A C camera takes any C-mount lens directly; a CS camera needs the 5 mm spacer from the section above.
3. Coverage. Compute the sensor diagonal and compare it with the printed image circle, as in the worked example.
4. Back focus. Open the iris, focus on a distant target, then lock the barrel. If distance stays soft while close range sharpens, the plane is off by a fraction of a millimetre — each barrel turn moves the lens 0.794 mm, so a 0.4 mm error is about half a turn — and a persistent offset is a shimming job, not a focusing one.

How does C-mount compare with an M12 board-lens thread?

An M12 (S-mount) board lens uses a much smaller M12 × 0.5 thread — 12 mm nominal diameter with a 0.5 mm pitch — and leaves back focus to the lens holder, where C-mount fixes thread, diameter and flange in the mount itself.

The two never mix directly: an adapter would have to shrink a 25.4 mm barrel into a 12 mm bore and rebuild a fixed flange inside a holder meant to be hand-adjusted, which is why board-level cameras stay on S-mount glass. Where the mount must survive vibration, the C-mount's larger thread and seating face hold calibration better; board cameras compensate by locking the M12 barrel with a set screw or adhesive. Choosing S-mount optics is its own procedure: M12 lens selection for board cameras: format, focal length and IR cut covers the optical side, Board camera lens mount thread sizes: M12 or M6 the thread sizes, and M12 lens thread pitch and focus shift: the physics of S-mount focus the focus mechanics.

The reverse adaptation does exist: a C-mount lens can ride on an M16 board camera when the adapter stack reproduces the flange plane — see C-mount lens on an M16 board camera: adapters and flange distance.

Which mount should you actually order?

Order C-mount when the camera holds the lens at the flange distance and the diagonal is 11 mm or more; add a 5 mm spacer only on a 12.5 mm CS camera; pick M12 board lenses only where board-level size outweighs the fixed flange.

• Machine-vision camera, C-mount: any C-mount lens whose printed image circle meets your sensor diagonal — machine vision lenses filter by image circle, focal length and sensor format.
• CS-mount camera: the same lens plus the nominal 5 mm adapter, or a CS-native lens.
• Board-level camera: M12 board lenses, chosen by holder height and back focus — Choose a board camera lens by working distance and field of view walks that decision.

Shortlist in the lens finder or let the lens configurator match lens to camera step by step. The rest of the mount arithmetic — flange tolerance, adapters, close focus — lives on the lenses topic page.