C-Mount

|K WONG

The C-Mount is a standardized, screw-thread mechanical interface used for connecting a camera lens to a camera body or other optical instrument. While originally developed for the cinematography industry, it has evolved to become the ubiquitous global standard for industrial, scientific, and medical imaging applications.

Summary

  • Type: Screw-thread mount.
  • Key Identification: 1-inch diameter with 32 threads per inch.
  • Primary Industries: Machine Vision, CCTV (legacy), Microscopy, 16mm Cinematography.
  • Key Advantage: Simplicity, robustness, and near-universal compatibility across industrial camera and lens manufacturers.

Technical Specifications

The mechanical specifications of the C-Mount are universally defined to ensure interchangeable compatibility between hardware from different manufacturers.

1. Thread Dimensions

The mechanical interface consists of a male screw thread on the lens and a matching female thread on the camera.

  • Nominal Diameter: 1 inch (approximately 25.4 mm).
  • Thread Pitch: 32 Threads Per Inch (TPI).
  • Unified Thread Standard (UTS) Designation: 1"-32 UN 2A (Male) / 2B (Female).

2. Flange Focal Distance (FFD)

This is the most critical optical measurement of the mount system. The Flange Focal Distance (often also referred to as "Flange-to-Film Distance" in legacy contexts) is the precise distance from the camera's mounting flange to the image sensor or film plane.

For a C-Mount, this distance is defined as: 17.526 millimeters (0.6900 inches).

Mechanical Specifications

These terms explain the physical mechanics of how lenses attach to optical instruments.

  • Flange Focal Distance (FFD): The critical distance from the camera's mounting flange to the image sensor or film plane. (Also called Flange-to-Film distance).
  • Thread Pitch: The distance between threads, often measured in Threads Per Inch (TPI) for Imperial or millimeters for metric (e.g., the 32 TPI of a C-Mount).
  • Bayonet Mount: A fastening mechanism consisting of a cylindrical male side with one or more radial pins (or tabs) and a female receptor with matching L-shaped slots. It is the modern alternative to screw-thread mounts like the C-Mount.

Optical Characteristics & Sensor Matching

When discussing C-Mount lenses, you inevitably have to discuss the sensors they project light onto.

  • Image Circle: The circular cross-section of light projected by a lens onto the image plane. A C-Mount lens typically has an image circle designed for sensors up to a 4/3-inch format (often smaller, like 1-inch or 2/3-inch).
  • Sensor Format: The physical size of the camera's image sensor. C-Mounts are typically paired with specific sensor formats.
  • Vignetting: The darkening of an image's corners. In the context of mounts, mechanical vignetting occurs when a C-Mount lens is placed on a camera with a sensor larger than the lens's maximum image circle.

Hardware and Accessories

These are the physical components used to modify or adapt C-Mount systems.

  • Spacer Ring (C/CS Adapter): A 5 mm thick threaded ring used to adapt a C-Mount lens to a CS-Mount camera by extending the Flange Focal Distance.
  • Extension Tube: Hollow rings placed between the camera and lens to increase the lens-to-sensor distance, changing the lens's focusing range to allow for macro (close-up) imaging.
  • C-Mount Adapter: A mechanical interface used to attach a C-Mount lens to a different camera mount (e.g., C-Mount to Micro Four Thirds adapter) or vice versa.

Common Applications

Because the standard has existed for decades, it is found in a wide variety of industries:

  • Industrial Machine Vision: The dominant standard for factory automation, robotics, high-speed inspection, and quality control cameras.
  • Microscopy (Scientific Imaging): The standard interface used to mount digital cameras onto microscope phototubes for capturing photomicrographs.
  • Surveillance (CCTV): Historically, the original standard for CCTV security cameras and lenses (now largely superseded by the related CS-Mount standard in security applications, though still common).
  • Legacy Cinematography: Originally designed for 16mm motion picture cameras (Bell & Howell first introduced the standard). It is still used by hobbyists and specialty filmmakers.

The C-Mount and CS-Mount Relationship

In modern imaging, the C-Mount is closely related to the newer CS-Mount standard. Understanding the relationship between these two is critical to achieving proper focus.

The CS-Mount Difference

The CS-Mount (Cinema Standard-Mount) was introduced to facilitate smaller, lower-cost cameras and lenses, primarily for the security market.

A CS-Mount possesses the exact same thread dimensions (1"-32 TPI) as a C-Mount. The only mechanical difference is the Flange Focal Distance.

  • C-Mount FFD: 17.526 mm
  • CS-Mount FFD: 12.526 mm (exactly 5 mm shorter)

Critical Compatibility Rules

Because the threads are identical, physical connectivity is always possible, but optical focus depends on the combination of lens and camera.

  1. C-Mount Lens on a CS-Mount Camera: COMPATIBLE. Because the lens requires more space from the sensor than the camera naturally provides, this works perfectly. To bridge the 5mm gap, you must use a 5mm C/CS spacer ring between the lens and the camera to achieve focus at infinity.
  2. CS-Mount Lens on a C-Mount Camera: INCOMPATIBLE. A CS-mount lens is designed to sit closer to the sensor. When placed on a C-mount camera body, the physical bulk of the camera mount prevents the lens from getting close enough to focus, resulting in an irrevocably blurry image. There is no simple adapter to make this combination work.