What Is a Telecentric Lens?

A telecentric lens is a compound lens system engineered so that its chief rays travel parallel to the optical axis. Because magnification is determined by chief-ray geometry rather than by object distance, the image of a part keeps the same size even when the part moves closer to or farther from the lens. In machine vision, this constant-magnification behavior eliminates the perspective error that makes ordinary lenses unreliable for dimensional measurement.

That single paragraph is the whole idea — but the engineering behind it, and the trade-offs that come with it, are what you need to understand before specifying one. This article explains how a telecentric lens works, how object-space, image-space and bi-telecentric designs differ, what specifications actually matter, and when a telecentric lens is worth the extra cost compared with a standard FA lens.

Why Ordinary Lenses Fail at Measurement

With a conventional photographic or FA lens, the chief rays from each object point converge toward the aperture stop at an angle. Move the object 1 mm closer to the lens, and its image becomes slightly larger. This is perspective (parallax) error, and it produces three very practical problems on a production line:

• Size drift. The same part measures differently depending on where it sits along the Z axis. A 10.00 mm shaft can read 10.03 mm one moment and 9.98 mm the next, purely because of fixturing variation.
• Shape distortion. Features at the edge of the field of view are seen obliquely. A round hole images as an ellipse, and the side wall of a deep bore becomes visible when the inspection really needs to see the bottom.
• Unstable calibration. Any magnification drift between parts or between fixtures forces recalibration — and recalibration costs line downtime.

Software cannot fully fix these effects. Sub-pixel edge detection and distortion correction help, but they cannot recover information that the lens never captured in the first place. That is why measurement-grade systems start with the optics, not the algorithm.

How a Telecentric Lens Works

The chief ray of an object point is the ray that passes through the center of the aperture stop. In a telecentric design, the aperture stop is positioned at the focal plane of the lens group in front of it. Any ray aimed at the center of that stop is refracted so that it emerges parallel to the optical axis. Once all chief rays are parallel, two things happen:

• Magnification is locked. Moving the object along the Z axis may blur the image, but it does not change the image size. Focus errors no longer become measurement errors.
• Viewing angle is fixed. Every feature in the field of view is seen from the same direction — the lens behaves like an orthographic measuring probe rather than a camera looking at a scene in perspective.

Which side of the lens is telecentric depends on where the stop sits, and that leads directly to the three telecentric types.

The Three Types: Object-Space, Image-Space, and Bi-Telecentric

For most factory-floor measurement tasks, an object-space telecentric design is sufficient and more affordable. A bi-telecentric design is the choice when the application also needs coaxial (through-the-lens) illumination, or when the highest measurement accuracy is required. POMEAS covers both families: the LTC15-110 1.5× object-space telecentric lens for compact measurement stations, the 1.0× bi-telecentric lens with 110 mm working distance for 1.1-inch-sensor metrology, and the LTCM2-110C 2× coaxial telecentric lens for reflective-surface inspection.

The Parameters That Actually Matter

Telecentric lens datasheets carry more numerically defined specifications than FA lens datasheets. These are the ones that decide whether a lens will measure accurately on your line:

• Magnification. Typically 0.2× to 2×. The field of view equals the sensor size divided by the magnification, so magnification and sensor format must be chosen together.
• Distortion. The geometric error that bends straight features. High-quality telecentric lenses hold distortion below 0.1%, and precision designs reach 0.05% or lower — one to two orders of magnitude better than a typical FA lens.
• Telecentricity. How close the chief rays are to truly parallel, usually specified as a maximum angle such as 0.1°. Low telecentricity is what keeps the image size stable when parts move in Z or sit at slightly different heights.
• Depth of field. Roughly 2–3× deeper than an FA lens of equal resolution. It is set by the F-number, the wavelength, and the allowable blur circle — stopping down trades resolution for depth.
• Working distance. The distance from the front of the lens to the object. Long working distances keep the optics away from moving machinery, at the cost of a larger front barrel.
• Sensor compatibility and mount. Telecentric lenses are designed for a maximum sensor format (for example 2/3″ or 1.1″) and usually ship in C-mount. Using a sensor larger than the design format puts the uncorrected corners of the sensor outside the optical field.

Telecentric Lens vs. Standard FA Lens

The physical reason behind the larger barrel is simple: to keep chief rays parallel over a given field of view, the front group must be at least as large as the field itself. A 40 mm field of view needs a front lens of roughly 40 mm — which is also why telecentric lenses become impractical for very large objects.

When to Use a Telecentric Lens — and When Not To

Choose telecentric when:

• you measure dimensions, positions, or angles and the result feeds an accept/reject decision;
• parts can sit at slightly different heights in the fixture, and the measurement must not change with them;
• you inspect deep features — bores, threads, connector pins — where side walls must not be visible;
• defect decisions depend on true defect size thresholds, not just defect presence;
• the same station must measure several part variants without recalibration.

Save the budget and use an FA lens when:

• the task is presence/absence detection, text or code reading, or robot guidance;
• the field of view is large relative to realistic lens diameters;
• the required accuracy is looser than the perspective error of a corrected FA lens plus software calibration;
• space or cost constraints dominate the project.

How to Select a Telecentric Lens in Five Steps

1. Fix the sensor and mount. Sensor format and C-mount come first, because every optical parameter is specified for a given format.
2. Compute magnification from the field of view. Magnification = sensor size ÷ field of view. Leave 10–20% margin for part placement tolerance.
3. Set the working distance. Include fixture clearance and any protective window between lens and part.
4. Match resolution to the sensor. The lens must resolve at least as many line pairs per millimeter as the sensor's Nyquist limit; otherwise the pixels, not the optics, will limit accuracy.
5. Buy accuracy where it counts. Convert the part tolerance into an allowable distortion and telecentricity budget, then pick the lens grade that satisfies it with margin — and pair the lens with a collimated (telecentric) backlight, because edge imaging quality depends on the illumination as much as on the lens.

For a worked example of this process, see the telecentric lens selection guide, or browse the full POMEAS telecentric lens family for magnifications from 0.2× to 2×.

Frequently Asked Questions

1. What does “telecentric” actually mean in a lens?

It means the chief rays — the rays passing through the center of the aperture stop — are parallel to the optical axis on one or both sides of the lens. Parallel chief rays in object space make magnification independent of object distance, which is the property measurement systems need.

2. Which type should I choose: object-space, image-space, or bi-telecentric?

For dimensional measurement without coaxial light, object-space telecentric is the cost-effective choice. If the system uses coaxial illumination or a beam splitter, or if uniform brightness across a large sensor is critical, choose a bi-telecentric design. Pure image-space telecentricity mainly benefits color sensors and beam-splitter optics and is rarely bought as a standalone feature.

3. Why are telecentric lenses larger and more expensive than FA lenses?

The front lens group must be at least as large as the field of view, and the design needs multiple precision lens groups to hold distortion and telecentricity at the 0.1% / 0.1° level. More glass, larger elements, and tighter tolerances all raise cost.

4. What measurement accuracy can I expect?

Accuracy depends on the whole chain — lens distortion, telecentricity, sensor resolution, calibration quality, and illumination. As a rule of thumb, a well-specified telecentric lens with proper calibration supports measurements at the 1–10 μm level on suitable fields of view; the exact budget should be calculated from the datasheet values for your magnification.

5. Will a telecentric lens work with my camera?

Check three things: the lens is designed for your sensor format or smaller, the mount matches (C-mount in most cases), and the lens resolution meets your sensor's pixel pitch. A lens designed for 2/3″ should not be paired with a 1.1″ sensor even if it screws on.

6. When should I avoid a telecentric lens?

Avoid it for very large fields of view (the front barrel grows with the field), for simple presence/absence or code-reading tasks, and for cost-sensitive projects where a calibrated FA lens plus software meets the accuracy target. Telecentric optics solve measurement problems; they are not a general upgrade for every camera.

Key takeaway: a telecentric lens buys you one thing ordinary optics cannot give — an image whose size does not depend on where the part is. If your inspection decision depends on absolute dimensions, that single property usually pays for the lens many times over in avoided false rejects and recalibration downtime.

Further reading: the test methods behind this article are documented in POMEAS Technical Reference when a standard lens is sufficient and when a telecentric lens is required (§§10.3 选型判据).