When Is a Telecentric Lens the Wrong Choice?

A telecentric lens is the wrong choice whenever the accept/reject decision does not depend on scale fidelity. Presence checks, OCR, defect screening, counting and robot guidance are decided by contrast and resolution, not by whether magnification holds across the depth of the part — and a conventional FA lens does that work at a fraction of the cost, size and working-distance constraint. The second common mismatch is field size: an object-side telecentric lens needs a front element at least as large as the object field, so a large field means a lens in that size class, with the mass and the illumination hardware that follow.

What telecentricity actually buys

Two properties, and only two. Magnification is defined by construction rather than by object distance, so a surface closer to the lens by h no longer changes scale by roughly h / WD — with a 300 mm working distance, a 5 mm step is about 1.7 % of scale on a fixed-focal lens and effectively zero on a telecentric one. And the chief rays stay parallel to the axis across the field, which removes the perspective occlusion band behind step walls and the shimmer that oblique rays produce on bores and edges. Distortion is also typically held below 0.1 %. None of these three extends depth of field, which still follows DOF ≈ 2 · F# · CoC · (1 + M) / M² and shrinks with 1 / M².

Where the size and the cost come from

The front element must cover the object field, so doubling the field diameter roughly quadruples the glass area, the barrel diameter and the mass. Working distance is the second driver, because the lens and stop geometry has to hold the telecentric condition over that distance. Specification is the third: tighter telecentricity, a larger image circle or lower distortion each add cost. This is why the honest way to specify starts from the measured tolerance and the smallest field that contains the feature, not from the largest part that might ever be inspected.

A three-question decision rule

1. Is the deciding quantity a dimension with a tolerance, or a presence/classification decision? Only the first needs scale fidelity.
2. Is the tolerance smaller than about five object-side pixels, or does part height vary by more than the depth of field you can afford? Either condition pushes towards telecentric.
3. Is the required field small enough that an object-side telecentric lens stays practical in size, mass and cost?

If the first answer is no, stop — a fixed-focal or FA lens is the correct specification. If the field is too large, split the requirement across two stations instead of oversizing one lens. The questions and answers below work through the individual cases, including what to check when a telecentric lens is already installed but the edges look poor.

Related reading: how a telecentric lens works, telecentric lens selection: FOV, WD, DOF, parts with different heights, and the telecentric lens range. Send us the tolerance and the field and we will say whether telecentric is the right answer.