Optics for industrial cameras: telecentric, fixed focal length, and 360° compared

Optics for industrial cameras: telecentric, fixed focal length, and 360° compared

The choice of optics for industrial cameras is one of the most decisive factors for image quality, measurement accuracy, and the overall reliability of a machine vision system.

Opto Engineering designs and manufactures three major families of machine vision lenses, each engineered for a specific application scenario:

• telecentric lenses, indispensable for high-precision dimensional measurement because they eliminate parallax error and maintain constant magnification;
• fixed focal length (entocentric) lenses, versatile and ideal for a wide range of inspection and quality control applications, typically with a C-mount;
• 360° view lenses, which make it possible to inspect the top, side walls, or internal cavities of cylindrical or hollow objects in a single acquisition.

This guide analyzes the technical characteristics, advantages, and typical applications of each type, providing practical criteria for selecting the lens best suited to your inspection process and for optimizing the performance of your vision system.

The role of the optics in an industrial vision system

In an industrial machine vision system, the lens is not an accessory to the camera but the true starting point of the design: it determines what the sensor sees, with what geometric fidelity, at what effective resolution, and with what robustness against variations in position.

This article presents, in a structured way, the three main types of industrial lenses offered by Opto Engineering®, telecentric, fixed focal length, and 360°view lenses, comparing them on optical principles, performance, and areas of use. The goal is to guide designers, OEMs, and system integrators toward the choice most consistent with their inspection, measurement, guidance, or quality control task.

To guide your reading, the article is organized around the following questions:

• Why is the choice of lens strategic in an industrial vision system?
• What are telecentric lenses, and when should they be preferred?
• What defines fixed focal length lenses, and in which applications are they ideal?
• What are 360° view lenses, and which applications do they solve?
• Telecentric vs. fixed focal length lenses: what criteria should you use to choose?
• How do you optimize industrial image resolution starting from the lens?

Why the choice of lens is strategic in an industrial vision system

In a machine vision system, overall performance depends not only on the camera but on the interaction between the CMOS sensor, pixel size, illuminator, and above all, the lens. It is the lens that projects the image of the object onto the sensor: a suboptimal choice introduces distortion, perspective errors, and parallax effects that no software algorithm can recover after the fact without a loss of accuracy.

Opto Engineering is recognized as one of the world leaders in telecentric technology and in specialty lenses with 360° views, with more than twenty years of experience in the design, manufacture, and support of machine vision components. Its offering spans the entire range of machine vision components: lenses, area-scan and line-scan cameras, LED illuminators, and imaging software.

The practical rule is simple: first define the vision task (measurement, surface inspection, code reading, robotic guidance, etc.), then identify the lens family that is most consistent with it, and finally size the sensor resolution, illumination, and field of view. This is where Opto Engineering's three major technology branches begin.

What telecentric lenses are and when they should be preferred

Telecentric lenses are designed so that the chief rays are parallel to the optical axis in object space (object-space telecentric) and, in the more advanced models, in image space as well (bi-telecentric). The result is behavior radically different from that of traditional entocentric lenses: magnification remains constant even as the object moves closer to or farther from the sensor, distortion is virtually nil, and parallax (perspective) error is neutralized.

This characteristic makes them the key component in any application that requires precision measurement and non-contact metrology. For three-dimensional objects, or those positioned with imperfect mechanical tolerances, an entocentric lens would introduce dimensional errors caused by perspective; with a telecentric lens, by contrast, the measured dimensions remain stable. Opto Engineering offers a broad range of telecentric and bi-telecentric lenses for area-scan and line-scan sensors, with classic, compact, and flat models as well as special configurations. The family also includes telecentric lenses with integrated liquid lenses (TCEL series), dual-magnification telecentric lenses (TCDP PLUS), and the compact CORE telecentric lenses, developed for measurement applications where space is critical, such as in the beverage and packaging sectors.

Typical applications include the dimensional control of machined turned parts; the measurement of profiles, gears, and electronic components; the inspection of flat-panel displays; the verification of the fill level of bottles; and the inspection of pharmaceutical caps and vials. Pairing them with a telecentric illuminator, such as the LTCLHP (or its compact version, the LTCLHP CORE), makes it possible to extend the depth of field and further improve optical accuracy.

What defines fixed focal length lenses and in which applications they are ideal

Fixed focal length lenses represent the most versatile, general-purpose family in the Opto Engineering catalog. They are machine vision lenses available for small-, medium-, and large-format sensors, with various focal lengths, apertures (f-numbers), and resolutions. Most of these lenses use the standard C-mount,the de facto standard of industrial machine vision, and cover sensor formats from 1/3″ up to 1.1″ and beyond.

The catalog includes series such as EN2MP, EN5MP, and EN-12RT, designed respectively for 2, 5, and 12 megapixel cameras, with typical focal lengths from 8 mm up to 75 mm and adjustable apertures. To these are added specialty lenses with integrated, remotely controlled liquid lens technology for fast, automatic focusing; lenses for SWIR and UV applications; lenses with Scheimpflug adjustment; and low-distortion macro lenses for close-up inspection.

The applications of these C-mount industrial lenses cut across many fields: surface inspection for detecting scratches, dents, contaminants, or cosmetic defects; the reading of 1D, 2D, and DataMatrix codes; presence/absence verification of components; label and print control; robotic guidance and pick-and-place applications; and the inspection of circuit boards as well as food, pharmaceutical, and cosmetic packaging.

Unlike telecentric lenses, these lenses introduce perspective, so they are not suited to precise metrology; it is also important to account for distortion (even though it is often calibratable) and to ensure that MTF and resolution are consistent with the pixel size of the chosen camera.

Their strength lies in the balance among optical performance, configuration flexibility, and cost. In vision systems, fixed focal length lenses are typically used for surface inspection applications, while telecentric lenses are selected when high accuracy in dimensional measurement is required. As demonstrated by application cases, Opto Engineering's 5 MP and 12 MP ITALA industrial cameras are paired with fixed focal length lenses for surface inspection and with telecentric lenses for metrology.

What 360° view lenses are and which applications they solve

360-degree lenses for inspection represent one of Opto Engineering's most distinctive innovations. They solve a structural problem in vision systems: the inspection of cylindrical or axisymmetric objects such as bottles, cans, vials, caps, containers, screws, or threaded holes, where defects can appear at any location along the surface and are predictable neither in position nor in orientation.

To ensure reliable inspection, it is therefore necessary to capture the entire 360° geometry in a single image, simultaneously covering the side surfaces, the top or bottom, and, where required, internal cavities. Without a complete view, the system would require multi-camera configurations or rotation of the part, with a significant increase in complexity, cost, and synchronization challenges.

The family comprises several lenses, based on the following main technologies:

• Pericentric lenses (PC and PCCD series): provide, in a single image, the top view and the outer side views of objects with diameters from 2 to 65 mm (PC series) and from 7.5 to 75 mm (PCCD series), ideal for inspecting caps and vial necks, reading DataMatrix codes, and pharmaceutical, beverage, automotive, and metallurgical inspections.
• Hole-inspection lenses (PCHIL series): optimized for the bottom and internal walls with a curved field and extended depth of focus, with an 82° viewing angle. The PCHIL3M model extends performance to sensors up to 1.1″ and is available both with manual focus and with integrated liquid lens technology for fast autofocus.
• Hypercentric lenses (HC series): allow inspection of the inside of cavities by observing from the outside through narrow openings such as bottle necks, cans, tubes, and threaded holes. They are also available in a nano version (HCN) for cavities from 0.75 mm to 10 mm in diameter.
• Multi-mirror lenses (PCPW, TCCAGE series): generate multiple views of the side surface in a single image, returning multiple side views within one frame.

The practical advantages of 360° view lenses are very concrete: a reduced number of cameras, no image-matching software between different images, a more compact system, and simple in-line integration because objects can pass beneath the lens without any need for mechanical rotation.

Telecentric vs. fixed focal length lenses: what criteria to use to choose

The comparison between telecentric and fixed focal length lenses is not settled by a ranking of quality, but by the consistency between the lens and the vision task. The two families solve different issues.

Telecentric lenses are the right choice when:

• the application requires dimensional measurement with tight tolerances and sub-pixel repeatability;
• the object has pronounced 3D geometry or is not perfectly positioned on the working plane;
• it is necessary to eliminate parallax effects and maintain constant magnification.

Fixed focal length (entocentric) lenses are the right choice when:

• the task is surface inspection, code reading, presence/absence, or cosmetic quality control;
• budget and space must be kept in check;
• perspective does not introduce critical error.

360° view lenses come into play when:

• side or internal surfaces need to be inspected in a single acquisition;
• you want to simplify the machine layout by replacing entire multi-camera systems with a single lens.

In real-world applications, these three families are used at different stations within the same vision system: a telecentric lens for high-accuracy dimensional measurement, a fixed focal length lens to capture qualitative information about surfaces, and a 360° view lens for the complete inspection of cylindrical geometries or cavities,all supported by high-resolution area-scan cameras.

How to optimize industrial image resolution starting from the lens

Optimizing industrial image resolution does not mean simply increasing the camera's megapixels: it means ensuring that every pixel of the sensor collects useful information. The system's effective resolution depends on the lens-sensor coupling, that is, on the resolving power of the lens relative to the pixel size of the CMOS in use.

A 12 MP sensor paired with an inadequate lens can be limited by the resolving power and the contrast transferred by the lens (MTF), producing an image that is nominally high-resolution but carries reduced useful information. Under such conditions, a 5 MP system correctly sized to the pixel size and optical magnification can deliver equivalent, or in some cases superior, performance.

Some practical criteria for selecting the lens in a vision system, consistent with Opto Engineering's approach:

• define the magnification as a function of the field of view (FOV) and the working distance, consistent with the geometry of the application;
• verify that the pixel size, in combination with the optical magnification, allows the smallest defect to be represented by a number of pixels sufficient for reliable analysis;
• choose lenses with a resolution (MTF) appropriate to the sensor's pixel size and sized for the actual format, avoiding vignetting, loss of sharpness at the edges, and lens-sensor mismatch;
• assess depth of field (DOF) and working distance, keeping in mind that sharpness and contrast are not uniform across the entire DOF and degrade at its extremes;
• design the illumination as an integral part of the system, in relation to the lens and the type of defect: telecentric or collimated illumination for transmission measurement (backlighting), diffuse or coaxial for surface inspection, directional to emphasize relief and texture.

For complex applications, Opto Engineering provides selection tools (webtools) that support the sizing of the lens-sensor pairing as a function of field of view, working distance, and required resolution. As a manufacturer of vision system components, the company works alongside the designer in defining the architecture, ensuring consistency among lens, sensor, and illumination, and providing software tools both for configuration and for image processing.

How to find your way in the selection

The three families of lenses for industrial cameras described here respond to different technical needs: they are not alternatives but complements.

Telecentric lenses remain the reference standard for non-contact metrology and high-precision dimensional measurement.

Fixed focal length lenses provide the flexibility needed for the majority of general inspections, quality control, and code-reading tasks.

360° lenses enable the complete inspection of cylindrical objects in a single image acquisition, including the side surface, the internal/external walls, and the top or bottom. This eliminates the need for multi-camera systems or rotation of the part, making the solution more compact, simpler, and less costly than traditional approaches.

The correct selection always arises from an integrated assessment of the vision task, object geometry, required resolution, working distance, and illumination and it is at this design stage that Opto Engineering's more than twenty years of experience in machine vision represents the most important added value for OEMs, system integrators, and Power End Users.