T5357: How to determine the camera's Resolution, Field of View and Focal Length

Article ID: T5357
Date: 1/30/13
Product: Vision Guide

Here are the basic steps for determining the physical setup of a vision system ->

First determine the acceptable positioning tolerance allotted for by the vision system. For example, let's say the application can accept up to +/-20 microns of variation in positioning tolerance. Note that this example assumes that all other sources of variation (tooling, robot repeatability etc…) are better then +/-20 microns.

a) Determine the required mm per pixel resolution for the camera. The positioning tolerance is +/-20 microns or 40microns total. Assuming an imaging deviation of 5 pixels, the required camera resolution is 0.04mm/5 pixels = 0.008 mm/pixel. Even though the vision tools are capable of sub pixel accuracy, it is always good practice to assume that there will be lighting variability. A very conservative assumption for pixel deviation resulting from part imaging/lighting is 5 pixels.

b) Determine the field of view that will be required to attain this resolution. Our "standard resolution" cameras have a 640 x 480 pixels resolution. For this example, 640 pixels x 0.008 mm/pixel = 5.12 mm and 480 pixels x 0.008 mm/pixel = 3.84 mm. The camera's required field of view (FOV) will be 5.12 mm by 3.84 mm. This field of view will attain the desired positioning tolerance.

c) Decide where you want to mount the camera. The distance from the camera to the object / feature of interest is called working distance.

d) Find out the "Image size" of the camera's CCD array. For the Sony XC-ES30, for example, the image size is 1/3 inch (3.6mm) CCD.

e) Knowing the object to lens distance, field of view and image size, you can calculate the lens' focal length as follows -

Focal length = (Working distance x Image Size) / (Y value of Field of View)

For example,

Positioning tolerance = 12mil or 0.3048 mm

Assumed pixel deviation = 2 pixels

Required camera resolution = 0.3048 mm/2 pixels = 0.1524 mm/pixel

Required Field of View = 640 pixels x 0.1524mm/pixel = 97mm and 480 pixels x 0.1524mm/pixel = 73 mm

Desired working distance = 38" or 965 mm

Image Size = 1/3" or 3.6mm

Focal length = (965mm x 3.6mm) /73mm = 47mm

A 50mm focal length lens can be used for this example.

Author of the Article: John Yett

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Last Updated
31st of January, 2013

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