What Are Lens Mounts and Why Do They Matter?
In machine vision and robotics, the lens mount is the mechanical interface between your lens and camera sensor. The mount type determines not just physical compatibility, but also optical performance, system size, and available lens options. Choose wrong, and you'll face everything from inability to focus to severe vignetting.
This guide covers the dominant mounts in embedded and industrial vision: M12 lenses (S-mount) for compact systems, CS-mount lenses for ITS and industrial cameras, C-mount lenses for professional machine vision, plus smaller M8 and M7 mounts for miniaturized applications. We'll compare specifications, explain compatibility, and help you select the right mount for your application.
M12 Mount (S-Mount): The Embedded Vision Standard
What is an M12 lens mount?
M12 mount, also called S-mount, uses a metric M12×0.5mm thread that screws directly into a board-level holder. Unlike C/CS mounts, M12 has no fixed flange focal distance – you achieve focus by adjusting the thread-in depth, then lock it with a set screw or adhesive.
Key Understanding
The MBFL of a lens is a fundamental constraint that determines how many optical elements and how large each much be. M12 lenses have a much shorter MBFL (e.g. 2.0mm) than C-Mount lenses which allow them to achieve comparable performance with smaller and fewer elements, resulting in lower cost and smaller form factor.
M12 Mount Specifications
- Thread Size: M12×0.5mm (12mm major diameter, 0.5mm pitch)
- Flange Focal Distance: Variable (typically 5-15mm range)
- Typical Sensor Coverage: 1/4" to 1/1.5" (lens dependent)
- Focus Mechanism: Thread depth adjustment with locking
- Common Applications: Embedded vision, robotics, drones, IoT cameras
M12 Lens Advantages and Limitations
Advantages: Smallest form factor, lightweight (typically 2-10g), cost-effective for volume production, wide FOV options (fisheye to telephoto).
Limitations: Manual focus adjustment required during assembly, less robust than C-mount for repeated handling, limited to smaller sensor formats, potential for focus drift without proper locking.
CS-Mount: Compact Industrial Standard "C-short" Mount
What is a CS-mount lens?
CS-mount uses the same 1"-32 UN thread as C-mount but with a shorter 12.5mm flange focal distance. This 5mm reduction allows for a shorter MBFL and therefore better IR correction than C-mount lenses, making CS-mount the preferred choice for Intelligent Transportation Systems (ITS), security cameras, and space-conscious industrial applications.
CS-Mount
CS-mount are common in ITS deployments (traffic monitoring, ANPR, toll collection) due to a shorter flange distance, allowing for better IR correction in comparable form factor.
CS-Mount Specifications
- Thread Size: 1"-32 UN (1 inch diameter, 32 threads per inch)
- Flange Focal Distance: 12.5mm (fixed)
- Sensor Coverage: Up to 4/3" format
- Focus Mechanism: Lens focusing ring + optional back focus adjustment
- Common Applications: ITS, security cameras, compact industrial vision
⚠️ Common Misconception
CS-mount is NOT the same as C-mount. While they share the same thread, the different flange focal distances mean you need adapters for cross-compatibility. A C-mount lens requires a 5mm spacer to work on a CS-mount camera.
C-Mount: The Machine Vision Workhorse
What is a C-mount lens?
C-mount is the classic machine vision standard with a 17.526mm flange focal distance. Developed in the 1940s for 16mm film cameras, C-mount offers the widest selection of industrial lenses, from ultra-wide to telephoto, with options for filters, aperture control, and precision focusing mechanisms.
C-Mount Specifications
- Thread Size: 1"-32 UN (same as CS-mount)
- Flange Focal Distance: 17.526mm (fixed)
- Sensor Coverage: Up to 4/3" format (some specialty lenses larger)
- Focus Mechanism: Precision focusing ring with locking screw
- Common Applications: Machine vision, microscopy, industrial inspection
M8 and M7: Miniature Mounts for Compact Vision
When space is at an absolute premium
For applications requiring extreme miniaturization, M8 and M7 mounts offer even smaller form factors than M12.
M8 Mount Specifications
- Thread Size: M8×0.35mm or M8x0.5mm (8mm major diameter, 0.35mm pitch)
- Sensor Coverage: Up to 1/3" (typically 1/4" or smaller)
- Lens Diameter: 10-12mm typical
- Applications: Medical devices, micro-robotics, wearables
M7 Mount Specifications
- Thread Size: M7×0.35mm (7mm major diameter, 0.35mm pitch)
- Sensor Coverage: 1/4" and smaller (typically 1/6")
- Lens Diameter: 8-10mm typical
Important: Sensor Size Limitations
M8 and M7 mounts are physically limited to smaller sensors. M8 typically maxes out at 1/3" sensors, while M7 is usually restricted to 1/4" or smaller. The small lens diameter constrains the maximum image circle and light-gathering ability.
Complete Specification Comparison
| 仕様 | M7 | M8 | M12 (S-Mount) | CS-Mount | C-Mount |
|---|---|---|---|---|---|
| Thread Type | M7×0.35mm | M8×0.35mm | M12×0.5mm | 1"-32 UN | 1"-32 UN |
| Thread Diameter | 7mm | 8mm | 12mm | 25.4mm (1") | 25.4mm (1") |
| Flange Focal Distance | Variable | Variable | Variable | 12.5mm | 17.526mm |
| Max Sensor Coverage | 1/4" typical | 1/3" typical | 1/1.2" typical | 4/3" format | 4/3" format |
| Typical Weight | 1-3g | 1-5g | 2-10g | 20-100g | 30-200g |
| Lens Diameter | 8-10mm | 10-12mm | 14-20mm | 25-40mm | 25-50mm |
| Cost Range | $3-$20 | $3-$20 | $5-$100 | $50-500 | $50-1000 |
Mount Compatibility and Adapters
Can you convert between mount types?
Mount conversion depends on the flange focal distance relationship and physical thread compatibility. Commonlands offers specific adapters to bridge between mount types where mechanically feasible.
✓ Direct Compatible
M12 lens → M12 holder
CS lens → CS camera
C lens → C camera
⚡ Adapter Available
C lens → CS camera
M12 lens → CS/C camera
✗ Not Feasible
CS lens → C camera
C/CS lens → M12 camera
Available Adapters from Commonlands
M12 to CS-Mount Adapter
The M12 to CS-mount adapter allows you to use M12 lenses on CS-mount cameras. This adapter includes an integrated M12 holder with the correct spacing to achieve proper back focus distance. Ideal for testing M12 lenses on industrial cameras or creating hybrid systems.
C to CS-Mount Spacer (5mm)
The C to CS-mount spacer is a precision 5mm ring that allows C-mount lenses to be used on CS-mount cameras. This simple adapter compensates for the FFD difference (17.526mm - 12.5mm = 5.026mm) and maintains full infinity focus capability. Essential for anyone mixing C and CS equipment.
Adapter Rule of Thumb
You can always adapt a longer FFD lens to a shorter FFD camera by adding spacers. Going the opposite direction requires relay optics or isn't feasible. For variable FFD mounts (M12, M8, M7), compatibility depends on having sufficient focus travel range in the adapter.
How to Select the Right Mount for Your Application
Decision flowchart for mount selection
-
Determine your sensor size and format
Check your sensor diagonal:- ≤1/4" (4.5mm): M7, M8, or M12 all viable
- 1/3" to 1/1.2" (6-13mm): M12 or CS/C-mount
- 1" to 4/3" (16-22mm): CS or C-mount required
-
Evaluate space constraints
M7/M8: Ultra-compact, under 12mm diameter
M12: Embedded systems under 25mm diameter
CS: Compact housings 30-50mm
C: Requires 40mm+ for lens and focus mechanism -
Consider optical requirements
Need ultra-wide fisheye? M12 has more options
Need motorized zoom for ITS? CS-mount and C-Mount are best
Need precise aperture control? CS-mount and C-Mount are best
Need smallest possible? M7/M8 for miniaturization -
Factor in mechanical robustness
High vibration environment? M12 lenses or ruggedized C-Mount lenses
Outdoor deployment? M12 lenses with ingress protection
Set-and-forget installation? M12 with adhesive lock
Frequent lens changes? C-mount for easy of install -
Match to camera interface
Board-level camera modules: Usually M12 or M8
ITS/traffic cameras: C-mount or CS-mount
Industrial GigE cameras: Usually C/CS
USB cameras: Typically M12 lens
Mount Selection by Industry
- Robotics & Drones: M12 dominates due to weight and size constraints
- ITS & Traffic: CS-mount for IR correction and large format
- Security & Surveillance: CS-mount for indoor, C-mount for outdoor PTZ
- Machine Vision: C-mount for flexibility and lens selection
- Medical Devices: M7/M8 for endoscopes, C-mount for microscopy
- Automotive: M12 for ADAS cameras, C-mount for inspection systems
- Wearables & IoT: M7/M8 for extreme miniaturization
Installation Best Practices
M12 Mount Installation
- Clean sensor glass and lens rear element with appropriate materials
- Thread lens into holder while monitoring live image
- Adjust for optimal focus across entire field
- Apply medium-strength threadlocker or UV adhesive
- Verify focus stability through temperature cycling if required
C/CS Mount Installation
- Verify mount type compatibility (C vs CS)
- Add 5mm spacer if mounting C-lens on CS-camera
- Thread lens fully, then back off 1/8 turn
- Adjust focus ring while viewing target at working distance
- Tighten focus lock screw without over-torquing
⚠️ Common Installation Mistakes
- Over-tightening M12/M8/M7 lenses causing thread damage or tilt
- Forgetting C→CS spacer resulting in inability to reach infinity focus
- Not accounting for temperature-induced focus shift in outdoor applications
- Using wrong threadlocker strength (permanent vs removable)
よくある質問
What is the thread size of an M12 mount?
M12 mount uses a metric M12×0.5mm thread. The "M12" indicates a 12mm major diameter, and "0.5" represents the 0.5mm thread pitch (distance between threads). This is different from the imperial 1"-32 UN thread used by C and CS mounts.
What is the difference between C-mount and CS-mount?
C-mount and CS-mount use identical 1"-32 UN threads but differ in flange focal distance (FFD). C-mount has 17.526mm FFD while CS-mount has 12.5mm FFD. This 5mm difference means you can use C-mount lenses on CS-mount cameras with a 5mm spacer, but CS-mount lenses cannot be used on C-mount cameras without optical correction.
What are M8 and M7 mounts used for?
M8 and M7 are miniature lens mounts for ultra-compact applications. M8 (8mm thread) typically supports sensors up to 1/3", while M7 (7mm thread) is limited to 1/4" or smaller sensors. These mounts are common in medical endoscopes, wearable cameras, and micro-robotics where every millimeter of space matters.
Can I adapt an M12 lens to a CS-mount camera?
Yes, you can use an M12 to CS-mount adapter to mount M12 lenses on CS-mount cameras. This adapter includes an M12 holder with proper spacing to achieve correct back focus. It's useful for testing M12 lenses on industrial cameras or creating hybrid systems.
What sensor sizes work with each mount type?
Mount compatibility by sensor size:
• M7: Up to 1/4" (typically 1/6")
• M8: Up to 1/3" (typically 1/4")
• M12: 1/4" to 1/1.2" typical
• CS-Mount: Up to 4/3" format
• C-Mount: Up to 4/3" format (some specialty lenses larger)
Always verify the lens's image circle covers your sensor diagonal to avoid vignetting.
Why is CS-mount preferred for ITS applications?
CS-mount dominates Intelligent Transportation Systems because it offers: compact camera housings for roadside cabinets, compatibility with motorized zoom lenses for variable coverage, standardized mounting for easy maintenance, and support for larger sensors (up to 4/3") needed for high-resolution ANPR and traffic monitoring.
What is flange focal distance (FFD)?
Flange focal distance (FFD) is the distance from the lens mounting flange to the sensor plane when focused at infinity. It's a critical specification that determines whether a lens can achieve proper focus on a given camera. C-mount has 17.526mm FFD, CS-mount has 12.5mm FFD, while M12, M8, and M7 have no fixed FFD (adjusted by threading depth).