Telecentric lenses field depth

Field depth is stated on our product documentation: for most of our TC series, the stated field depth is the overall field depth at F/# 8.

The magnification of a telecentric lens always remains constant regardless the change of the working distance. Therefore we may consider that the best focus plane is located in the middle of the field depth, or in other words field depth is symmetrical to the best focus plane.

At the borders of the field depth the image can be still used for measurement but, to get a very sharp image, only half of the nominal field depth should be considered.

Field depth is a complex parameter: it depends on the magnification, F/#, wavelength, pixel size and, last but not least, on the sensitivity of the edge extraction algorithm in use. For this reason there is no objective nor standard way to define it: it's a subjective parameter.

A simple rule of thumb to calculate field depth:

field depth = (WFN * p * k) / (M * M)

where

M = magnification
WFN = working F/#
p = pixel size (in micron)
k = application specific parameter

The k parameter depends upon the type of application. For telecentric measurement applications, a reasonable k value is 0.008, while for defect inspection k should be set at about 0.015.
For certain magnification and working F/#, the bi-telecentricity of our lenses yields superior field depth values.