The Complete Guide to Fixed Focal Length Lenses (FA Lenses) for Machine Vision

In the world of industrial automation and quality control, machine vision systems have become indispensable. At the heart of these systems lies a critical component that’s often overlooked: the lens.

While cameras and software get most of the attention, the wrong lens choice can undermine even the most advanced vision system.

This comprehensive guide will walk you through everything you need to know about Fixed Focal Length Lenses (FA Lenses) for machine vision applications.

What is a FA Lens?

A FA lens (Factory Automation lens) is a specialized fixed focal length lens designed specifically for machine vision and industrial automation applications. Unlike consumer camera lenses, FA lenses are engineered to deliver consistent, high-precision imaging performance in demanding industrial environments, with features such as low distortion, high resolution, robust construction, and precise mounting systems to ensure accurate and reliable automated inspection results.

FA lenses differ from consumer or even professional photography lenses in several important ways:

1. They prioritize consistent, measurable performance over artistic qualities
2. They’re built to maintain precise calibration despite vibration and temperature changes
3. They offer superior resolution and contrast for detailed inspection tasks
4. They’re designed for 24/7 operation in industrial environments

💡 It’s worth noting that “FA lens” and “fixed focal length lens” are virtually identical concepts in the machine vision industry. The term “FA lens” is more commonly used in Japanese and Korean markets, while in North American and European countries, “fixed focal length lens” is the more familiar terminology for the same type of optics.

What is a Fixed Focal Length Lens?

A fixed focal length lens (also called a prime lens) has a single, non-adjustable focal length, as opposed to a zoom lens that offers variable focal lengths. In machine vision applications, fixed focal length lenses are preferred because they provide superior optical performance, greater reliability, and more consistent results without the complexity, size, and potential calibration issues associated with zoom mechanisms.

The simplicity of fixed focal length lenses offers several advantages for industrial applications:

• Higher light transmission (larger maximum apertures)
• Better edge-to-edge sharpness
• Fewer moving parts that could fail
• More compact size for tight installation spaces
• Better optical performance for the price

Key Features of FA Lenses

High Resolution

Modern machine vision cameras use high-resolution sensors with tiny pixel sizes, some below 2μm. FA lenses must resolve these small pixels clearly, which requires exceptional optical quality. Top-tier FA lenses can resolve details down to the pixel level across the entire image, not just in the center.

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Low Distortion

Distortion – where straight lines appear curved in images – can severely impact measurement accuracy. Quality FA lenses typically offer distortion below 0.5% or even 0.1% for metrology applications, ensuring that what the camera “sees” accurately represents reality.

Large Aperture

FA lenses often feature large maximum apertures (low f-numbers like F1.4 or F1.8) for several reasons:

• They allow shorter exposure times, reducing motion blur in high-speed applications
• They provide brighter images in light-limited environments
• They help maximize the system’s overall light sensitivity

However, in most precision applications, lenses aren’t used at their maximum aperture but stopped down slightly to optimize image quality and depth of field.

Aperture Comparison: F1.4 vs F2.8

Images captured under identical conditions:

• Same light source
• Equal exposure time
• Comparable image quality settings

The visual difference shown below demonstrates how aperture size affects image brightness and depth of field while maintaining consistent testing parameters.

Compact Design

Space constraints are common in industrial settings. FA lenses are designed to be compact while delivering the required optical performance, allowing for flexible installation in tight production environments.

Robust Construction

Unlike consumer lenses, FA lenses are built to withstand industrial conditions:

• Metal housings resist impacts and provide thermal stability
• Locking screws prevent focus and aperture shifts from vibration
• Sealed designs protect against dust and moisture
• Industrial-grade materials withstand temperature fluctuations

Standardized Mounts

Most FA lenses use standardized mounting systems, with C-mount being the most common. This ensures compatibility across different cameras and simplifies system integration and maintenance.

Advantages of FA Lenses in Machine Vision Systems

Improved Inspection Accuracy

The low distortion and high resolution of FA lenses enable vision systems to measure, recognize, and locate objects with precision. Even a small improvement in lens quality can significantly enhance detection accuracy, particularly for applications requiring sub-pixel precision.

Enhanced Environmental Adaptability

Industrial environments present challenging conditions: vibration, temperature fluctuations, limited space, and varied lighting. FA lenses are designed specifically to maintain performance despite these challenges.

Increased Inspection Speed

The large maximum apertures of FA lenses allow for shorter exposure times, which means cameras can capture clear images of fast-moving objects. This directly translates to higher throughput in production environments.

Consistent Long-Term Performance

Unlike consumer-grade optics that may drift or degrade over time, quality FA lenses maintain their optical performance over years of continuous operation, ensuring that vision systems deliver consistent results without frequent recalibration.

Common Applications of FA Lenses in Machine Vision Industry

Quality Inspection

In manufacturing, FA lenses enable vision systems to automatically identify and classify quality issues such as surface defects, dimensional deviations, or assembly errors. These systems can inspect hundreds of parts per minute with consistent accuracy that surpasses human capabilities.

For example, in electronics manufacturing, FA lenses help identify microscopic solder joint defects, component misalignments, or PCB errors that would be invisible to the naked eye.

Positioning and Guidance

The precision imaging provided by FA lenses allows vision systems to accurately locate objects and guide robotic operations. This capability is crucial in:

• Pick-and-place operations
• Part alignment for assembly
• Robot guidance in unstructured environments
• Bin picking applications

The low distortion characteristics of FA lenses ensure that coordinate measurements from images accurately reflect real-world positions.

Dimensional Measurement

When equipped with proper FA lenses, vision systems can perform non-contact dimensional measurements with accuracies approaching traditional metrology equipment. This is particularly valuable for:

• In-line 100% inspection of critical dimensions
• Measuring fragile or hot parts that can’t be contacted
• Complex profile measurement that would be difficult with physical probes
• High-speed measurement of moving parts

Color Recognition and Verification

In applications where color accuracy matters, specialized FA lenses (often used with specific filters or lighting) can enhance color differentiation. This is particularly important in:

• Pharmaceutical pill identification
• Food quality grading
• Color code verification
• Print quality inspection

How FA Lenses Work in Machine Vision Systems

A FA lens forms part of a complete imaging system that typically includes:

1. Illumination: Controlled lighting that highlights features of interest
2. Lens: The FA lens that focuses light from the subject onto the sensor
3. Camera: The digital camera with a sensor that converts light to electronic signals
4. Processing hardware: Computers or embedded systems that analyze the images
5. Software: Algorithms that extract relevant information from the images

The lens plays a critical role in determining what information is available to the system. A high-quality FA lens ensures that subtle details are preserved and accurately represented, while a poor lens can introduce distortions, blur, or other defects that make reliable inspection impossible.

How to Choose the Right FA Lens for a Machine Vision System

Selecting the appropriate FA lens involves several key considerations:

1. Working Distance and Field of View

First, determine:

• How large an area needs to be captured in each image (field of view)
• How far the lens must be from the subject (working distance)

These factors, along with the camera’s sensor size, determine the required focal length. Longer focal lengths provide:

• Greater working distances
• Narrower fields of view
• Potentially higher magnification

Shorter focal lengths offer:

• Wider fields of view
• Shorter working distances
• More depth of field

2. Resolution Requirements

Consider:

• The smallest feature that must be detected
• The required measurement accuracy
• The camera’s pixel size

As a rule of thumb, you need at least 2-3 pixels to detect a feature, and 10+ pixels for accurate measurement. Working backward from these requirements helps determine the resolution needed from your lens.

3. Aperture and Depth of Field

Depth of field refers to the range of distances that appear acceptably sharp in an image. Factors affecting depth of field include:

• Aperture setting (smaller apertures/higher f-numbers increase depth of field)
• Focal length (shorter focal lengths generally provide more depth of field)
• Working distance (greater distances typically increase depth of field)

For subjects with significant depth, you may need to prioritize depth of field over absolute resolution.

4. Lighting Conditions

Consider the available light and the exposure time requirements:

• For high-speed applications, larger apertures may be necessary
• For applications with intense lighting, smaller apertures often provide better optical performance
• For fluorescent lighting, exposure timing may need to be synchronized with the light’s flicker cycle

5. Environmental Factors

Evaluate the operating environment:

• Temperature range and fluctuations
• Presence of vibration
• Dust or moisture exposure
• Available mounting space

These factors may influence the mechanical requirements for your lens.

6. Budget Considerations

While high-end FA lenses offer superior performance, they also come with higher prices. Consider:

• The economic impact of potential inspection errors
• The expected service life of the system
• The criticality of the application
• The cost-benefit ratio of incremental performance improvements

Sometimes a mid-range lens is sufficient, while other applications justify premium optics.

Comparing FA Lenses to Other Lens Types

FA Lenses vs. Consumer Camera Lenses

While consumer lenses might seem like an economical alternative, they typically lack:

• The resolution needed for machine vision
• The mechanical stability required in industrial environments
• Low distortion for measurement applications
• Consistent batch-to-batch performance
• Long-term support and availability

FA Lenses vs. Telecentric Lenses

Telecentric lenses are specialized FA lenses that eliminate perspective distortion, making them ideal for precision measurement. However, they:

• Are significantly larger than standard FA lenses
• Cost substantially more
• Have limited fields of view relative to their size
• Provide benefits only for certain applications

FA Lenses vs. Macro Lenses

Macro lenses are designed for high magnification of small objects. While they share some characteristics with FA lenses, they:

• Often lack the working distance needed in industrial settings
• May not offer the same level of distortion control
• Typically aren’t designed for industrial durability
• May prioritize different optical characteristics

Maintenance and Care for FA Lenses

To maintain optimal performance from FA lenses:

Regular Cleaning

• Use compressed air to remove dust particles
• Clean optical surfaces with lens tissue and proper lens cleaning solution
• Never use aggressive solvents or abrasive materials
• Avoid touching optical surfaces with bare fingers

Proper Storage

• Keep lens caps on when not in use
• Store in low-humidity environments
• Avoid extreme temperature fluctuations
• Use desiccant in storage cases for long-term storage

Periodic Inspection

• Check for loose screws or mounting hardware
• Verify that focus and aperture locks remain secure
• Inspect for any signs of damage or contamination
• Confirm that optical performance hasn’t degraded

Recalibration

For critical applications, consider:

• Annual recalibration of the entire vision system
• Documentation of lens performance over time
• Replacement before performance deteriorates below acceptable levels

Future Trends in FA Lens Technology

The field of FA lenses continues to evolve with several notable trends:

Higher Resolution Support

As camera sensors move toward higher resolutions with smaller pixels, lens manufacturers are developing FA lenses capable of resolving these tiny pixels while maintaining other performance characteristics.

Specialized Designs for New Sensors

The growth of non-visible imaging (infrared, ultraviolet, multispectral) is driving development of specialized FA lenses optimized for these wavelengths.

Compact, High-Performance Designs

Advancements in optical design and manufacturing are enabling more compact lenses with better performance, benefiting space-constrained applications.

Integration with Intelligent Cameras

As smart cameras become more prevalent, FA lenses are being designed to integrate more seamlessly with these all-in-one systems.

Conclusion: The Critical Role of FA Lenses in Machine Vision

While often overshadowed by sensors and software, FA lenses play a fundamental role in determining the capabilities and reliability of machine vision systems. The right lens can make previously impossible inspections routine, while an inadequate lens can undermine even the most sophisticated vision system.

By understanding the unique features, selection criteria, and applications of FA lenses, engineers and system integrators can build more effective vision systems that deliver consistent, accurate results in demanding industrial environments.

When evaluating FA lenses, remember that optical performance is just one dimension—mechanical stability, long-term reliability, and suitability for the specific application are equally important considerations. The initial investment in quality optics typically pays dividends throughout the life of the vision system through improved accuracy, reduced false rejects, and consistent performance.

Whether you’re designing a new vision system or upgrading an existing one, giving proper attention to lens selection will help ensure that your system meets its performance goals now and in the future.

Looking for high-quality FA lenses for your machine vision application? Explore our selection of industrial-grade fixed focal length lenses designed specifically for demanding inspection environments.