Selecting a telecentric lens is not simply a matter of choosing the highest resolution or the largest sensor coverage. The correct optical configuration depends on the relationship between the object, camera sensor, field of view, magnification, working distance, and inspection accuracy.

For machine vision engineers and system integrators, choosing the lens early in the system design can help avoid mismatched components, insufficient field of view, limited working distance, or unnecessary optical costs.

This guide explains the key parameters to consider when selecting a telecentric lens for precision measurement, AOI, dimensional inspection, and industrial machine vision.

1. Start With the Inspection Object

Before selecting a telecentric lens, define the object and the actual inspection requirement. The lens should be selected around what needs to be measured or detected.

• Object size
• Required field of view
• Smallest feature to inspect
• Required measurement accuracy
• Object height variation
• Available installation space
• Object surface and reflectivity
• Required inspection speed

A small mechanical component requiring dimensional measurement may require a very different optical configuration from a large PCB requiring component or edge inspection.

2. Field of View: How Much of the Object Must Be Captured?

Field of View (FOV) is the physical area captured by the imaging system. The FOV should be large enough to contain the complete inspection area while still providing sufficient pixel coverage for the smallest feature of interest.

A practical starting point is:

For example, if an inspection area is 80 mm wide, designing for exactly 80 mm may leave little tolerance for part positioning or fixture variation.

FOV should also account for:

• Part positioning tolerance
• Conveyor movement
• Fixture accuracy
• Multiple objects in one image
• Required inspection region

3. Magnification: Connecting the Object to the Sensor

Magnification describes how the object is represented on the camera sensor. A simplified starting relationship is:

For example, if a camera has a 14.4 mm sensor width and needs to capture a 72 mm object width:

This provides a starting point for lens selection. Actual lens specifications, sensor format, optical design, and usable image circle must also be verified.

Why Magnification Matters

Too low: The inspection feature may occupy too few pixels.

Too high: The required FOV may not fit on the sensor.

For precision inspection, magnification should therefore be considered together with sensor resolution and feature size.

4. Sensor Size and Image Circle

The telecentric lens must adequately cover the camera sensor. Common machine vision sensor formats include:

• 1/1.8"
• 1.1"
• 1.2"
• 2/3"
• 4/3"
• APS-C
• Full Frame
• Large-format sensors

For line-scan and high-resolution applications, larger image circles may also be required.

A lens designed for a smaller sensor may result in vignetting, insufficient image coverage, or reduced image quality toward the edges.

5. Working Distance: More Than Just Installation Space

Working Distance (WD) describes the specified distance between the lens and the object plane. Working distance is especially important when integrating optics into an automated machine.

Consider working distance together with:

• Mechanical integration
• Lighting placement
• Part accessibility
• Protection from moving components
• Available depth of field
• Overall machine dimensions

A longer working distance can be useful when the object is inside a machine, mechanical components surround the inspection area, or additional space is needed for illumination.

However, longer WD does not automatically mean better optical performance. Working distance should be selected together with magnification, resolution, DOF, and mechanical requirements.

6. Resolution: Match the Lens to the Camera

A high-resolution camera does not automatically produce a high-resolution inspection image. The optical system must be capable of resolving the detail captured by the sensor.

When selecting a telecentric lens, consider:

• Sensor pixel size
• Camera resolution
• Required feature size
• Lens MTF
• Working wavelength
• Magnification
• Object contrast

For high-resolution machine vision systems, the camera and lens should be treated as one imaging system, rather than selected independently.

7. Telecentricity: What Level Do You Actually Need?

Not every application requires the same degree of telecentric performance. For measurement-oriented applications, object-space telecentricity can help reduce changes in apparent size caused by object position along the optical axis.

This becomes particularly useful when:

• Part height varies
• Objects cannot be positioned at exactly the same Z height
• Dimensional measurement is required
• Edge location is critical
• Repeatability is important

Telecentricity should still be considered alongside the actual depth of field and object-space tolerance of the application.

A telecentric lens is not a substitute for proper mechanical positioning or calibration.

8. Depth of Field: How Much Z Variation Can You Accept?

Real production systems rarely have perfectly flat objects positioned at exactly the same height. Parts may have thickness variation, mechanical tolerances, conveyor movement, or fixture variation.

This makes depth of field (DOF) an important selection parameter.

A larger DOF can help maintain acceptable image quality across a range of object positions. However, DOF should not be evaluated independently.

Define the required image quality and measurement tolerance first, then select the optical configuration accordingly.

9. Don't Forget Illumination

Lens selection and lighting design should be considered together. Even a high-performance telecentric lens cannot compensate for inadequate illumination.

For some precision applications, a telecentric lens + telecentric illumination configuration can provide more controlled object-plane illumination.

10. A Practical Telecentric Lens Selection Workflow

For system integrators, the following sequence can simplify the optical selection process.

Telecentric Lens Selection Checklist

Common Telecentric Lens Selection Mistakes

Telecentric Lens Selection for Different Applications

How to Select the Right Telecentric Lens for Your System

There is no single "best" telecentric lens for every machine vision application. The correct configuration depends on the relationship between:

For system integrators, defining these parameters before selecting the lens can make optical design more predictable and reduce the risk of component mismatch.