Introduction

For machine vision engineers, security camera integrators and AI vision device buyers, focal length is the top parameter when picking industrial optical lenses. Most newcomers confuse fixed lens focal length with varifocal lens parameters, and fail to match proper lenses for PCB cameras, ITS traffic devices and factory inspection systems.

Simply put, a fixed lens features non-adjustable built-in optics, and its focal length is fixed during manufacturing. This article simplifies core concepts, mount classification, practical selection tips and common questions about fixed lens focal length. As a professional China-based industrial vision lens manufacturer, TOWIN Lens provides full-series M12, C and CS mount fixed lenses for global clients, with customized design and fast global shipment services.

1. Core Definition: Fixed Lens & Its Focal Length

1.1 What Is an Industrial Fixed Lens?

A fixed focal lens (fixed lens) is an optical lens without zoom adjustment structure. Users cannot change its viewing angle after installation. It only supports iris adjustment to adapt to light changes, featuring compact structure, low distortion and stable imaging quality. It is widely used in 24/7 running vision equipment, different from adjustable varifocal zoom lenses.

TOWIN mainstream fixed lens types cover miniature board lenses, machine vision MV lenses and surveillance IR corrected lenses, compatible with mainstream 5MP sensors like IMX335.

1.2 What Is the Focal Length of a Fixed Lens?

The focal length of a fixed lens refers to the distance from the lens optical center to the camera image plane, measured in millimeters (mm). It decides two key imaging effects directly:

• Shorter focal length: Wider field of view, suitable for large-scene shooting and close-range detection
• Longer focal length: Narrow viewing angle, higher magnification, for distant target detail capture

Three core mount standards define fixed lens application scenarios on the market: M12/S-mount, C-mount and CS-mount, with different flange focal distances for matched cameras.

2. Fixed Lens Focal Length Classification & Mount Features

Combined with TOWIN’s product layout, industrial fixed lenses are divided into three focal ranges, matching exclusive mounts and application scenarios without complex data comparison:

2.1 Short Focal Fixed Lens (2.6mm - 8mm)

Mainly M12/S-mount miniature fixed lenses with M12×0.5 thread. Designed for small PCB embedded cameras, indoor panoramic surveillance, automotive vision and close-range product inspection. TOWIN optimizes low-distortion coating for short focal models to reduce barrel distortion for industrial use.

2.2 Medium Focal Fixed Lens (10mm - 25mm)

Universal focal size compatible with C-mount and CS-mount cameras. Balanced viewing angle and imaging clarity, ideal for general machine vision detection, IP security cameras and roadside basic monitoring. Most 5MP high-definition fixed lenses for IMX335 sensors fall into this range.

2.3 Long Focal Fixed Lens (35mm & above)

Mainly C-mount industrial fixed lenses, adopted for long-distance scene capture. Perfect for highway ITS license plate recognition, remote precision dimension detection and outdoor long-range monitoring, exclusive for professional industrial vision projects.

Key Mount Difference Note

C-mount flange focal distance is 17.526mm, while CS-mount is 12.526mm. The same focal length lens will generate slight imaging difference when matched with different camera mounts. M12 mount is ultra-small size only for mini embedded vision equipment.

3. Practical Guide: How to Choose Fixed Lens Focal Length

Follow 4 simple rules to pick the right fixed lens focal length for your project, no professional optical knowledge required:

• Judge by working distance: Close-range work below 0.5m chooses 2.6-8mm short focal lenses; 3m+ long-distance scenes choose 12mm+ medium/long focal fixed lenses.
• Judge by shooting scope: Need wide full-scene coverage → short focal lens; Need tiny target detail capture → long focal lens.
• Match camera sensor size: 1/2”, 1/3” small sensors match M12 fixed lenses; 2/3” large industrial sensors match C/CS mount fixed lenses to avoid image dark corners.
• Match usage scenarios: Mini AI camera = M12 pinhole fixed lens; Factory inspection = C-mount fixed MV lens; Day-night surveillance = IR corrected CS fixed lens.

If you cannot confirm focal length specs, contact TOWIN optical team via [email protected] for free one-on-one lens matching solution.

4. FAQs

Q1: Can I adjust the focal length of a fixed lens?

No. Fixed lenses have fixed internal optical components, focal length cannot be changed manually. If adjustable viewing angle is needed, varifocal lenses are alternative options.

Q2: Will lens mount affect fixed focal length performance?

Mount thread will not change original focal length value, but flange distance affects final imaging effect. C-mount lenses need a spacer to adapt to CS-mount cameras.

Q3: Are all short focal fixed lenses distorted?

No. TOWIN low-distortion M12 fixed lenses adopt multi-layer optical coating, controlling distortion within qualified range for precision industrial inspection.

Q4: What fixed focal lenses fit IMX335 5MP camera sensors?

Medium focal CS/M12 fixed lenses are the best match. Check qualified models on M12/S-Mount Lenses category on TOWIN official website.

5. Advantages of Fixed Focal Lenses

Compared with varifocal zoom lenses, fixed industrial lenses own irreplaceable strengths for mass industrial deployment:

• Higher imaging sharpness and stable definition for long-term automated detection
• Lower optical distortion, more suitable for measurement and defect detection work
• Solid integrated structure, fewer spare parts, low failure rate
• Cost-effective for bulk procurement, fast installation and replacement
• Professional IR coating optional, stable day-night imaging for surveillance cameras

Conclusion

The focal length of a fixed lens is the core index to decide viewing angle and shooting range. To sum up: pick short focal fixed lenses for wide close-range scenes, medium focal lenses for general vision use, and long focal lenses for distant detail monitoring. Meanwhile, match M12, C or CS mount based on your camera interface.

TOWIN Lens is a reliable industrial machine vision lens manufacturer in Xiamen, China, supplying customized fixed focal lenses, standard stock lenses with fast selection and worldwide delivery. Browse full product lines viaC-Mount Lenses and CS-Mount Lenses pages, or send your project requirements to [email protected] for professional optical support.

TOWINLENS - Professional Industrial Lenses Manufacturer | https://www.towinlens.com/