Telecentric Lens Depth of Field: Definition and Calculation Methods

Telecentric Lens Depth of Field: Definition and Calculation Methods

The depth of field (DOF) of a telecentric lens is a critical parameter, as it determines the range within which images remain sharp. The size of the DOF depends on multiple factors, including lens magnification, f-number, wavelength, pixel size, and the sensitivity of the edge detection algorithm used by the customer. For most telecentric lens series, the stated DOF is typically based on an overall f-number of 8.                                              

Depth of Field (DOF) Calculation Formula Diagram

Depth of field (DOF) can be used in measurement applications, where it is generally larger than that required for defect detection, and the image contrast must be as high as possible. A simple DOF calculation formula is as follows:

DOF = (Working F-number × Pixel Size × Application-specific Parameter) / (Magnification × M)

Where:
M represents magnification,
WFN denotes working F-number,
P stands for pixel size (μm),
K is the application-specific parameter.

The application-specific parameter depends on the type of application. For telecentric measurement applications, a reasonable value for the application-specific parameter is 0.015, while for defect detection applications it should be set to approximately 0.008.

It should be noted that DOF is a subjective parameter with no objective or standardized method to define it. Furthermore, the actual DOF is also influenced by other factors such as lens design, manufacturing quality, and operating environment.

In telecentric optical imaging, due to the dual-sided telecentric focusing characteristics, the lens exhibits relatively good depth of field (DOF). For example, when calculating the DOF of the bi-telecentric lens 2364 using the given parameters (magnification=0.38, pixel size=4.4μm, application-specific parameter=0.015, F-number=8) in the formula, the resulting DOF is approximately 31.55mm.

It should be noted that the DOF calculation formula provides only an approximate value, and the actual DOF may vary due to various factors. To obtain more accurate DOF values, it is recommended to perform actual measurements or use professional DOF measurement tools.

Songsheng Optoelectronics Telecentric Achromatic Scanning Objective Lens Diagram

Songsheng Optoelectronics has independently developed a coaxial temperature-measuring laser processing telecentric achromatic scanning objective lens specifically for machine vision laser precision machining requirements. This galvanometer scanning focusing system integrates coaxial imaging functionality and is specially designed for galvanometer scanning processing systems with high-precision positioning requirements. The CCD observation image is perfectly coaxial with the laser beam focus. When paired with F-theta lenses and illumination light sources, it enables "what you see is what you get" machine vision positioning laser processing.

Conventional fθ scanning lenses (field lenses), lacking achromatic design, require extensive calibration work to achieve coaxial vision through software correction. In contrast, telecentric achromatic scanning lenses fundamentally resolve these issues at the hardware level, eliminating the need for complex software-based positional alignment corrections while delivering superior positioning accuracy.

These lenses are primarily applied in machine vision laser processing applications including laser welding, laser marking, laser cutting, and laser resistor trimming.

Regarding the common perception that telecentric lenses inherently provide greater depth of field (DOF), this assertion is not entirely accurate. The actual DOF depends on multiple factors including lens magnification and f-number. While telecentric lenses offer distinct advantages such as minimal distortion and large field of view, they do not automatically guarantee greater DOF. Therefore, lens selection should be based on comprehensive evaluation of all relevant parameters and performance characteristics according to specific application requirements.