Depth of Field Calculation for Telecentric Lenses

Depth of Field Calculation for Telecentric Lenses

Depth of field (DOF) is a critical parameter in cameras, as it determines the range of sharp focus in an image. In telecentric optical imaging, DOF is also a frequently mentioned parameter, influenced by lens magnification, f-number, wavelength, pixel size, and the sensitivity of the edge extraction algorithm used by the customer. For most telecentric lenses, the specified DOF is based on an f-number of 8. DOF can be used for measurement applications, usually larger than defect detection, and requires the highest possible image contrast.

A simple rule for quickly calculating DOF is as follows:

• DOF = (Working F-number * Pixel Size * Application-specific Parameter) / (Magnification * Magnification)
  • M = Magnification
  • WFN = Working F-number
  • P = Pixel Size (microns)
  • K = Application-specific Parameter

The application-specific parameter depends on the type of application. A reasonable value for telecentric measurement applications is 0.015, while for defect detection, it should be about 0.008. For a given magnification and working f-number, since the focus is telecentric, the DOF of our lens is better.

Let's take an example to calculate the DOF for a dual telecentric lens model 2364 using this formula: Here is the data from the dual telecentric lens model 2364 product catalog:

• M = Magnification = 0.38
• P = Pixel Size = 4.4μm (depending on the camera used)
• K = 0.015 (Application-specific Parameter)
• F/N = F-number = 8
• WFN = Working F-number = (1 + Magnification) * F-number = (1 + 0.138) * 8 = 9.104

The DOF can be calculated as:

• DOF = (Working F-number * Pixel Size * Application-specific Parameter) / (Magnification * Magnification)
• DOF = (9.104 * 4.4 * 0.015) / (0.138 * 0.138) = 31.55 mm

However, the DOF value in the product catalog for the dual telecentric lens is 35 mm.

How to measure the DOF for a lens:

1. Adjust the working distance between your lens and the object to the correct position.
2. Once your frame is in focus, use software to measure your object.
3. Now move the object closer to and further from the lens while observing the software measurement results.
4. You can see that within an allowable range, your image frame doesn't change. This range is what we call the depth of field range.