# 【Telecentric lens】Calculation of depth of field for telecentric lens

In: Lens Wiki On: Wednesday, April 1, 2020 Hit: 2231

Depth of field is an important parameter in optical photography, and its size determines the clear image range. Depth of field is also a frequently mentioned parameter in telecentric optical imaging, and its size depends on the lens ratio, aperture number, wavelength, pixel size, and the sensitivity of the edge extraction algorithm used by the customer. For most telecentric lenses, the stated depth of field is the overall depth of field with an aperture of 8. Depth of field can be used for measurement applications. It is usually larger than the depth of field for defect detection, and the contrast of the image must be as high as possible.

For this reason, the product record of easton optics states that "the depth of field boundary, the image can still be used for measurement, but in order to get a very clear image, only half of the nominal depth of field should be considered". The depth of field is very difficult to define in terms of parameters: it depends on the multiplier, aperture number, wavelength, pixel size, and the sensitivity of the edge extraction algorithm used by the customer. For this reason, there is no objective or standard way to define it: it is a subjective parameter.

For a simple rule, a quick way to calculate the depth of field is as follows:

Depth of field = (operating aperture * pixel size * application specific parameter)/(magnification * M magnification)

M = magnification

WFN = number of operating apertures

P = pixel size (micron)

K = application specific parameters

Application-specific parameters depend on the type of application. A reasonable application-specific parameter value for telemetry applications is 0.015, while defect-detection application-specific parameters should be set to approximately 0.008. For certain magnification and operating aperture, the depth of field of our lens is better because of the double focus distance

Below, we take the calculation of depth of field of dual-telecentric lens 2364 as an example to illustrate the calculation formula of depth of field of telecentric lens:

The following is the product catalog data of dual telecentric lens 2364:

M= magnification =0.38

P= pixel size =4.4um (depending on your camera)

K=0.015 (application specific parameter)

F/N = number of apertures =8

WFN= operating aperture == (1+ magnification)* focal length ratio = (1+ 0.138)*8 = 9.104

Depth of field =(operating aperture * pixel size * application specific parameter)/ (magnification * magnification) =(9.104 * 4,4 * 0,015)/(0.138 * 0.138) = 31.55 mm

However, the depth of field data on the dual telecentric lens catalog is 35mm.

As you can see, using a formula with our actual product volume of the above data is slightly different, in fact I gave to your formula is a method of calculating data (many, in fact, the data applications cannot be judged according to the formula). The depth of field actually not difficult, so if you like, you can measure on your own.

The method of measuring the depth of field of a lens is as follows:

1) adjust the working distance between your lens and the measured object to the correct position;

2) once your picture is in focus, use software to measure your object;

3) now move the object closer to the lens and then away from the lens to see the result of your software measurement;

4) you can see that your image will not change within the allowable range, which is what we call the depth of field range. 