Telecentric Lenses (Machine Vision)

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Whether you are looking for a solution for precise measurement tasks, high reproducibility or demanding inspection processes – our experts will help you select the right telecentric lens. Together, we will find the optimal combination of magnification, working distance, sensor size and illumination for your application.

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From the initial idea to the finished machine vision solution: we accompany you through every phase of the project. Our experts will help you select the right telecentric lens, advise you on sensor technology, illumination and optical compatibility, and help you optimally tailor systems to your application. Benefit from technical advice, feasibility analyses and practical integration experience.

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What is a telecentric lens?

A telecentric lens is specially designed so that the chief rays are parallel to the optical axis in object space, image space, or both. This is achieved by placing the aperture stop at the front focal plane (object-space telecentric; entrance pupil at infinity), the rear focal plane (image-space telecentric; exit pupil at infinity), or both (bi-telecentric).

In an object-space telecentric lens, the image magnification is essentially independent of object distance within the specified telecentric range and depth of field. This suppresses perspective (parallax) errors and enables true-to-scale dimensional measurements. Image-space telecentric lenses deliver near-normal chief-ray incidence on the sensor, which reduces vignetting and color shifts on modern small-pixel sensors. Bi-telecentric lenses combine both benefits and provide the highest measurement stability for demanding metrology.

Telecentric lenses are used in industrial machine vision when measurement accuracy and reproducibility are crucial. They suppress perspective (parallax) errors typical of endocentric optics and maintain nearly constant magnification. As a result, measurements remain accurate despite moderate variations in object height or position. In addition, metrology grade telecentric lenses are typically optimized for extremely low geometric distortion, further improving measurement fidelity.

Guide: Selecting the right telecentric lens – selection criteria for your application

Telecentric lenses suppress perspective (parallax) errors and keep magnification nearly constant across the intended working range, delivering accurate metrology with minimal geometric distortion. This makes them ideal for precise, repeatable measurement and inspection tasks in industrial machine vision. This guide explains the key selection criteria and shows you how to use the corresponding filters in the STEMMER IMAGING product finder.

telecentric lenses for industrial machine vision

Field of view (FOV)

The field of view is the maximum object area that must be imaged onto the sensor. In telecentric imaging, it’s primarily determined by sensor size and chosen magnification of the telecentric lens (FOV ≈ sensor size ÷ magnification). The larger the FOV, the larger the front lens of the lens must be in order to to avoid vignetting and maintain uniform illumination. Allow for an additional small margin to accommodate part offset and size tolerances.

Note: Large objects such as complete printed circuit boards, molded parts or packaging, are usually imaged with low magnification telecentric lenses, covering larger fields of view. Small workpieces or finely structured components such as plug contacts, wires or microelectronics require a smaller FOV and higher magnification. The smaller the FOV, the smaller the front lens required. Conversely, as FOV increases, the lens diameter generally grows significantly to preserve telecentricity, brightness, and edge uniformity.

Product filter: Telecentric lenses are specified by a fixed magnification and image circle. Using your sensor format and required field of view plus margin, select the magnification that maps that field onto the sensor. Then, verify that the lens image circle fully covers the sensor.

Smallest structure and pixel size

Determine the smallest feature you must detect or measure. As a practical rule, plan for 3-5 camera pixels across that feature so edges are clearly defined. You can achieve this either by increasing magnification or by using a camera with smaller pixels (or both).

Note: Typical industrial cameras use pixels of ~2–5 µm; for very fine structures or sub-micrometre work, 1–3 µm pixels are common. Check that the lens can support your sensor’s resolution: we specify a minimum (recommended) pixel size the lens can resolve at its nominal F-number and working distance. Consider that stopping down increases depth of field but also diffraction.

Product filter: Use the “Minimum pixel size” filter to shortlist lenses that meet your sensor’s pixel size (set the filter to a value ≤ your camera pixel). If you are near the limit, prefer lenses with a smaller stated minimum pixel size or plan to run at a faster F-number.

Sensor size and number of pixels

Sensor size comes from pixel size × pixel count per axis (width and height); the diagonal follows from those dimensions. This size, together with magnification, determines how large an object area can be imaged on the sensor. Make sure the lens fully illuminates the sensor: its image circle must be ≥ the sensor diagonal to avoid vignetting and edge fall-off.

Note: Image sensor size in industrial machine vision range from 1/2" (≈ 8 mm diagonal) to 2/3" (≈ 11 mm) and 1" (≈ 16 mm) to 35 mm full-frame sensors. The larger the sensor, the larger the image circle of the lens must be. C-mount lenses typically illuminate sensor formats up to 1.1″; larger sensors require M42, M58 or F-mounts.

Product filter: In the product filter, you can select the "sensor size" (diagonal). This allows you to narrow down the results to lenses then shortlist lenses whose image circle covers the sensor.

Magnification (image scale)

Magnification is the ratio of image size on the sensor to the actual object size. In practice, you select it considering your sensor format and the required field of view. Higher magnifications allow for resolving finer details, but reduce the field of view (FOV).

Note: For large objects or coarse inspection features, magnifications below 0.5x are typically sufficient, as the field of view remains correspondingly large. Magnifications between 0.5x and 2x are suitable for many general measurement and inspection tasks, while values from 2x to 5x are used for small components such as connectors or conductor tracks. Very high magnifications (>5x) are mainly used for microscopy or microstructures. Note that as the magnification increases, both the field of view and the usable depth of field decrease (at a given F-number), and larger front apertures may be required to maintain resolution and uniform illumination.

Product filter: Set the "Magnification" attribute in the product filter to the calculated value. The portfolio covers magnifications from approximately 0.03x to 10x.

Working distance

The working distance (WD) is the nominal object distance at which a telecentric lens meets its specified magnification, telecentricity, and MTF. Many models are factory-set to a nominal WD with a small adjustment range; best performance is achieved close to this value. Take into account the available installation space, lighting access, and any required safety clearances.

Note: Short working distances of 10-50mm suit for compact setups and provide higher effective NA, improving resolution and light throughput. Medium distances of 50-150mm are common in many industrial inspection systems and offer sufficient space for lighting and handling. Long working distances of 150 mm or more are used for large parts or safety gaps. As distance increases, the object-side NA decreases, which reduces the resolution and light throughput – stronger illumination or longer exposure may be necessary.

Product filter: Use the "Working distance" filter to display only lenses with a suitable nominal working distance, and verify the allowed adjustment range for your setup.

Aperture (F-Number)

The F-Number controls light throughput and depth of field, and diffraction-limited resolution. Lower F-Numbers allow more light to pass through, but yield shallower depth of field. Stopping down increases DoF at the cost of diffraction softening. In telecentric setups, the effective F-number increases with magnification, so higher magnifications usually need brighter lighting or longer exposure.

Note: Small F-Numbers (e.g. F4–F8) provide high light intensity and resolution, but require precise focusing as the depth of field becomes shallow. Medium apertures (F8–F16) are practical compromise as they extend depth of field with a slight reduction in high-frequency MTF. Very high aperture numbers (≥F16) should be used only when the illumination is sufficiently strong and DoF dominates the resolution requirements, as diffraction effects can impair image quality.

Product filter: In the product filter, narrow the options down by F-number to balance exposure and depth of field, and check the recommended minimum pixel size of the lens at the chosen F-number.

Physical Length

The physical length is the overall barrel length from the mount face to the lens front. It determines whether the optics fits your mechanical setup and, together with the working distance, sets the total standoff to the part and the clearance for lighting.

Note: Physical length of the lens is driven mainly by image circle (sensor size), nominal working distance, required numerical aperture, and optical architecture (bi-telecentric designs are typically longer). Large fields of view often require bigger front groups that can extend the barrel. Adding coaxial illumination modules, filter holders, or protective windows increases the overall length and weight which is a critical consideration to ensure rigid support.

Product filter: Use the Length attribute to match your mechanical envelope. Also check housing diameter and weight, and verify that lens length + adapters + working distance fits your installation with room for illumination.

Lens Mount

The lens mount (C-Mount, M42, M58, Nikon F-Mount, etc.) must match the camera. Confirm what your camera natively supports (or which adapters are approved) and mind flange focal distance requirements.

Note: C-Mount telecentric lenses is the most widely used in machine vision projects and typically covers sensor formats up to approximately 1.1". For larger sensors and higher resolutions, mounts with larger clear apertures such as M42 or M58 or Nikon F-Mount are used. Larger mounts allow for a larger image circle and prevent vignetting, but are usually associated with larger and heavier optics. Thread pitches vary by manufacturer (e.g., M42×1 or M42×0.75, M58×0.75), so check compatibility. If using adapters, ensure the flange focal distance and clear aperture remain adequate.

Product filter: Select the appropriate mount under "Camera mount optics" to display only compatible lenses, and verify the lens image circle meets or exceeds your sensor diagonal to avoid edge shading.

Types of telecentricity

Telecentric lenses are available in three variants: object-side telecentric (chief rays parallel in object space: nearly constant magnification over the specified telecentric range), image-side telecentric (chief rays strike the sensor near-normal, constant angle of incidence on the sensor, and bi-telecentric (telecentric on both sides for maximum precision). Select the type of telecentricity according to your application.

Note: Object-side telecentric systems are suitable for measurement tasks where objects height or position varies, e.g. workpieces on conveyor belts. Image-side telecentric lenses are used when a constant angle of incidence is required (e.g. colorimetry multispectral work, or small-pixel sensors with microlenses). Bi-telecentric optics are the first choice for high-precision measurements (e.g. in wafer inspection, pin checks or dimensional metrology where scale and chief-ray angle must both be tightly controlled.

Product filter: Use the Telecentricity type attribute (object-side / image-side / bi-telecentric) based on your application needs.

Illumination

Telecentric imaging benefits most from telecentric (collimated) backlighting in transmitted-light setups. Parallel rays produce crisp, high-contrast silhouettes with minimal penumbra (partially lit transition zone at an edge in backlit images), and the object-side telecentric lens accepts only near-axial rays. This stabilizes edge position even when part height varies.

For reflective/opaque parts, pair the telecentric lens with coaxial/on-axis illumination (via a beamsplitter or coaxial module). Light travels along the optical axis and specular returns from flat surfaces are directed back into the lens, yielding uniform brightness and suppressing shadows. This setup is ideal for wafers, polished metals, and printed features.

Note: Match the illuminator’s emitting area to your FOV with margin. Make sure the clear aperture of any coaxial module meets or exceeds the lens front aperture to avoid vignetting. Choose wavelength to suit the sensor/filters, add polarizers to cut glare when needed, and plan strobing if you run higher F-numbers or magnifications. Verify working distance compatibility between the telecentric backlight and the lens.

Product filter: Select Coaxial/On-axis modules for best performance with reflective parts. The portfolio also includes telecentric lenses for illumination. In Illumination section, select collimated illumination. Check emitting area, clear aperture, working distance, and controller options to meet exposure and throughput targets.

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Our experts will be happy to assist you in selecting the right telecentric lens for your application – from the initial assessment to the final system integration.

Frequently Asked Questions (FAQ)

Why do most lenses in industry use a C-Mount?

C-mount is the de-facto standard in machine vision because it balances precision, size, and cost. The standardised 1-32 UN (2A/2B) thread and fixed support dimension of 17.526 mm ensure a precise, defined interface between suppliers, allowing cameras and lenses to be easily combined.

The C-mount provides a compact, threaded, and mechanically stable interface for telecentric lenses and is commonly used with sensors up to about 1.1. Its wide availability and cost-effective production ecosystem have made it the industry standard, especially for small to medium sensor formats.

When is it worth using a telecentric lens instead of a standard lens?

A telecentric lens is always worthwhile when measurement accuracy, dimensional stability and reproducibility are critical. Unlike endocentric (standard) lenses, telecentric lenses can maintains nearly constant magnification, supressing perspective parallax errors, even if the object position or height changes slightly.

Telecentricity is particularly valuable in dimensional metrology and gauging (e.g., pins, shafts, threads, connector leads), inspection of rotationally symmetric or finely structured parts, and measurements in multilayer/transparent materials. The performance is improved further especially with a telecentric backlight providing crisp, threshold-independent edges. For angle-sensitive sensors or colorimetry, image-side telecentric designs help by delivering near-normal chief-ray incidence across the field. Choose a telecentric lens when you need to detect small dimensional deviations or measure features on components with varying heights or multiple planes with high precision.

What advantages does a bi-telecentric lens offer over object-side or image-side telecentricity?

A bi-telecentric lens is designed to be telecentric on both the object and image sides. This means that the chief rays run parallel to the optical axis in both sides. It combines the benefits of object-side telecentricity (nearly constant magnification over the specified telecentric range, suppressing parallax from object-height variation) and image-side telecentricity (near-vertical ray incidence on the sensor, reducing shading, color shifts, and sensitivity to small sensor misalignment).

Compared to object-side or image-side only design, bi-telecentric lenses deliver the highest imaging precision, measurement stability and uniformity across the field. It is the preferred choice for high-precision measurement and inspection tasks that require absolute dimensional accuracy across the entire field of view, such as in industrial metrology or automated inspection systems with the highest tolerance requirements.

How does the choice of illumination (e.g. telecentric vs. diffuse) affect the measurement result?

Lighting has a significant impact on image quality and measurement accuracy by setting edge contrast and glare. Telecentric illumination (transmitted/backlight) sends near parallel rays through the part. This creates crisp silhouettes with a very small penumbra, so edge position is less sensitive to threshold settings and small height variations. This is ideal for exact dimensional and shape measurements.

Diffuse illumination distributes the light from many directions. It reduces reflections on shiny or irregular surfaces, and it is well suited for surface checks and defect detection. The trade-off is slightly softer edges, which can increase measurement uncertainty.

In industrial machine vision, the choice of lighting therefore always depends on the application. For precise metrology and measurement tasks, we recommend combining a telecentric lens with telecentric illumination. If backlighting is not possible on opaque reflective parts, consider coaxial on-axis lighting with polarization.

What factors determine measurement accuracy in practice?

Accuracy is set by the whole imaging chain. On the optics side, you need sufficient contrast at the required spatial frequencies, and low geometric distortion. In addition to the resolution of the optical system, a flat image field and stable reproduction ratio, which is achieved through telecentricity, play a central role. Depth of field at the chosen F-number must cover the part tolerance without pushing diffraction too far.

Equally important are the sensor size and pixel pitch of the camera. Pixel pitch must provide enough sampling per feature, with good SNR and stable response. Illumination drives edge contrast and repeatability. Illumination should be uniform and repeatable over the object area, with glare and stray light under control and timing matched to the exposure.

Permanent mechanical stability closes the loop. Rigid mounting, consistent working distance, and precise alignment prevent drift; vibration and thermal changes must be managed.

In practice, measurement accuracy is achieved through the precise coordination of all components – optics, camera, lighting and mechanical stability. Reproducible results with minimal measurement deviation are achieved only when these factors are properly coordinated.

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