A telecentric lens measures parts at their true size. Its field of view is fixed by its magnification. Choose the camera sensor, or enter its size and pixel size. Then enter the size of the part and a margin. The calculator returns the strongest stocked magnification that still covers the part. It also shows the field of view, the size of one pixel on the part and every stocked magnification side by side. Not sure you need a telecentric lens at all? Read when you need a telecentric lens for gauging.
How the telecentric calculation works
Take the stocked magnification at or below the result. A stronger lens would cut off the edges of the part. Then check the working distance and depth of field of that lens against the machine. The front of the lens must also be larger than the field of view.
Worked example
A 20 × 15 mm part with a 10 % margin needs a field of at least 22 × 16.5 mm. A 2/3-inch, 5 MP sensor is 8.45 × 7.07 mm with 3.45 µm pixels:
- width: 8.45 / 22 = 0.384x, height: 7.07 / 16.5 = 0.428x, so at most 0.384x
- strongest stocked magnification below that: 0.3x
- field of view: 8.45 / 0.3 × 7.07 / 0.3 = 28.2 × 23.6 mm
- pixel on the part: 3.45 µm / 0.3 = 11.5 µm (0.0115 mm)
Lighting and the rest of the system
For sharp silhouettes, pair the lens with a collimated backlight. Parallel light gives the cleanest edges on thick or round parts. For surface features, use a coaxial light that fits the front of the lens. The camera sets the pixel size: compare sensors among area scan cameras. To pick camera, lens and light that fit together, build the system. For a standard lens instead, use the lens focal length calculator.
Questions
How do I choose the magnification of a telecentric lens?
Divide the sensor size by the field of view you need. The field must hold the whole part plus a margin, so take the smaller of sensor width ÷ field width and sensor height ÷ field height, then choose a stocked magnification at or below it. A higher magnification shows more detail but a smaller field.
Why does a telecentric lens have a fixed field of view?
A telecentric lens has a fixed magnification and only collects light that travels parallel to its axis, so it sees exactly the area in front of its front lens divided by the magnification. There is no zoom or focal length to change it: a different field needs a different magnification.
What is the pixel size on the part?
The camera's pixel size divided by the magnification. With 3.45 µm pixels and a 0.5x lens, one pixel covers 6.9 µm on the part. Measurements are usually reliable to a fraction of a pixel with edge detection, so this sets the measuring resolution.
Object-side or bi-telecentric?
Both keep the size of the part constant as it moves within the depth of field. A bi-telecentric lens is also telecentric on the camera side, so small focus and sensor-position changes do not change the size either; it is the choice for the highest measuring accuracy.