M12 board lenses

How to choose an M12 board lens

M12 lenses (also called S-mount lenses) screw into a holder with an M12 x 0.5 thread mounted on the camera board. They are small, light and easy to build in, which makes them the standard optic for board-level cameras, embedded vision, compact code readers and smart cameras. The trade-off is fewer adjustments: most have a fixed aperture and no focus ring, so the choice has to be right on paper.

1. Sensor format and image circle

Match the lens to the sensor format: the image circle must cover the sensor diagonal. M12 lenses in this range cover formats from 1/4" (about 4.5 mm diagonal) through 1/2.7", 1/2.5", 1/2" and 1/1.8" (about 9 mm), with a few for 2/3". A lens for a 1/2.7" sensor on a 1/1.8" sensor leaves dark, soft corners. A lens with a larger image circle than the sensor works, but the angle of view is narrower than the lens's headline figure, which is quoted for its largest format.

2. Focal length and angle of view

Focal lengths run from about 1 mm fisheye lenses to 25 mm, with a few longer lenses up to 70 mm. Because the sensors are small, short focal lengths already give wide views: a fisheye lens here reaches about 210° diagonal, while 8-16 mm lenses suit inspection at a few hundred millimetres. Work it out with focal length = sensor width x working distance / field of view width (using the sensor width, not the diagonal), or use the focal length calculator. Angles of view in the specifications are usually listed per sensor format; use the row for your sensor.

3. Resolution, aperture and chief ray angle

Resolution classes run from VGA to 16 MP. The lens should be rated at or above the camera's resolution, keeping in mind that small pixels (2-3 µm) need sharper optics than large ones. Most M12 lenses have a fixed iris between about F1.4 and F2.8, with some at F5.6 for more depth of field. Check the chief ray angle (CRA) against the sensor's microlens design: a mismatch causes colour shading and brightness fall-off toward the corners, even when the lens is sharp.

4. Distortion

Wide M12 lenses have strong barrel distortion, often 10-20% or more on wide angles and far more on fisheyes. That is fine for viewing and detection, and software can correct it after calibration, but for measurement choose a low-distortion design - some here are specified below 0.1%. Distortion is listed on each product page where the maker states it.

5. IR filter and spectrum

Many board cameras need an IR-cut filter in the lens or holder for correct colour under daylight; for 850 or 940 nm illumination you need the opposite. Lenses here come with a visible-light IR-cut filter, a 940 nm filter, a dual band-pass filter or no filter, and some are IR corrected (day/night) so focus holds when lighting switches between visible and infrared. Check what the camera board already has before ordering.

6. Mechanics and environment

Check the total length and the back focal length against the holder height and the space in the housing. Focus is set by turning the lens in the holder and fixing it with a lock nut or adhesive. For outdoor, vehicle or wash-down use, some lenses here are sealed to IP68 or IP69K, with metal barrels and water-repellent front coatings. A few use M6, M8, M10, M16 or CS thread instead of M12; check the thread, not just the focal length.

Typical applications

• Board-level cameras built into machines, kiosks and instruments.
• Spare and alternative lenses for code readers and smart cameras with an M12 lens mount (check that the housing still seals).
• Robot and AGV cameras, where low weight matters.
• Vehicle cameras: rear view, surround view and driver monitoring, using the sealed and IR-corrected lenses.
• Wide-angle monitoring of a machine area with fisheye lenses.

Common mistakes

• Reading the angle of view for a larger format and getting a narrower view on a small sensor.
• Pairing a lens with a strong IR-cut filter with 850 nm lighting, or a lens without one with a colour sensor.
• Ignoring the chief ray angle and getting colour shading at the corners.
• Not locking the lens after focusing, so vibration shifts focus over weeks.
• Using a wide-angle lens for measurement without calibrating out its distortion.

Questions

Is an M12 lens the same as an S-mount lens?
Yes. S-mount is the common name for the M12 x 0.5 thread.

Can I use an M12 lens on a C-mount camera?
With an M12-to-C adapter, yes, but the M12 lens's small image circle only covers small sensors. For C-mount cameras, a C-mount machine vision lens is usually the better choice.

Why do M12 lenses have no iris?
To stay small and keep the focus fixed. Light is controlled with exposure time, gain and the lighting; choose a slower fixed aperture (higher F-number) if you need more depth of field.

Where are lenses for CCTV and security cameras?
Varifocal, fisheye and zoom lenses for security cameras are under CCTV lenses. To pair a lens with a camera, start in the configurator.