Telecentric lenses

How to choose a telecentric lens

A telecentric lens keeps the image size of the part constant when the part moves closer or farther within the depth of field, and removes perspective, so edges and holes are measured at their true size. Choose by magnification, sensor and working distance.

1. Magnification = sensor size / field of view

The field of view is the sensor size divided by the magnification. Pick the magnification that fits the part with a small margin, or let the telecentric lens calculator work it out for your sensor and part:

2. Sensor format

The lens must be rated for the camera's sensor format or larger, or the corners go dark. Larger sensors need larger (and longer) lenses with F-mount or M58 / M72 mounts.

3. Object-side or bi-telecentric

Object-side telecentric lenses keep the magnification constant as the part moves. Bi-telecentric lenses are telecentric on the camera side too, so small focus changes and sensor tilt do not change the size either: the choice for the highest measuring accuracy.

4. Working distance, depth of field, front size

Check the working distance against the space in the machine, and the depth of field against how much the part height varies. The front of the lens must be larger than the field of view - a telecentric lens only sees what fits in front of it.

5. Lighting

For silhouettes, pair the lens with a collimated backlight: parallel light gives the sharpest edges. For surface features use a coaxial light that fits the lens.