When to Use a Telecentric Lens in Machine Vision Applications?
When to specify a telecentric lens in machine vision
Telecentric lenses come up regularly in our work — not as a first choice, but as the right choice for a specific set of problems. This page explains when we specify them, based on applications we have actually delivered, and addresses a common misconception that leads some projects to specify them unnecessarily.
What makes a telecentric lens different
A standard lens introduces perspective error — objects closer to the lens appear larger than objects further away. In most vision applications this doesn't matter. But when you need to measure accurately between features that sit at different distances from the camera, perspective error becomes a real problem. A telecentric lens eliminates this. The optical path is designed so that only rays parallel to the optical axis reach the sensor, meaning magnification stays constant regardless of depth. The result is that features at different depths appear at the same scale, and measurements between them are accurate.
When we specify a telecentric lens
Dimensional measurement where accuracy matters
On a project inspecting drilled holes in a precision-machined block, we used a telecentric lens to measure hole diameter, check for burrs, and verify positional accuracy of each hole relative to a calibrated XY table. The combination of accurate diameter measurement and positional data — with the camera calibrated to the machine coordinate system — required the distortion-free imaging that a telecentric lens provides.
Measurement across features on different planes
On an orthopaedic implant inspection application, we needed to measure between three reference points that sat on slightly different planes. The tolerance was ±0.25mm. A standard lens would have introduced perspective error between the planes, making accurate measurement impossible. A telecentric lens solved this directly. The same principle applied on a surgical instrument assembly station, where a specific length of blade had to protrude from its holder within a very tight tolerance. The vision system measured the blade tip on one plane and the edge of the holder on another, feeding the measurement to an actuator that drove the assembly to the correct position. The field of view was small, the measurement was cross-plane, and the accuracy requirement was demanding. Telecentric was the only viable choice.
Seeing through apertures
A telecentric lens is also useful when you need to see through a small aperture — a drilled hole, the head of a cable tie — without the perspective distortion that a standard lens would introduce. The parallel optical path allows a clean, undistorted view through the aperture.
A common misconception — depth of field
We regularly encounter the assumption that telecentric lenses offer greater depth of field than standard lenses — sometimes the assumption is that depth of field is effectively infinite. This is not the case. All things being equal, a telecentric lens offers no better depth of field than a telephoto lens of equivalent focal length. When telecentric is necessary, it is necessary because of the measurement accuracy requirement — not because of depth of field. If depth of field is the primary concern, there are other approaches worth considering first.
When telecentric isn't necessary
If your application doesn't involve measurement between features at different depths, and perspective distortion isn't affecting the accuracy of what you're trying to do, a standard or telephoto lens will often perform just as well at significantly lower cost and in a smaller physical package. The cost of a telecentric lens doesn't go away because the budget is tight. If the application demands one, it demands one. But we always evaluate whether it's genuinely required before specifying one — because in applications where it isn't, you're paying for capability you don't need.