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I am working on a project based on machine vision . Wide angle lens with high resolution pinhole camera is being used.

Working distance : Distance between Camera and object .

The resolution will be nearly 10MP. The image size may be 3656 pixel width and 2740 pixel height. The project requirements are as mentioned below

  1. My working distance must be nearly 5metres.
  2. The camera needs to be tilted at an angle of 13 degree.

To avoid lens distortion in camera I do camera calibration using OpenCV.

Below mentioned are my doubts pertaining to this camera calibration

  1. Since the working distance is 5 meters, should the camera calibration too be done with the same distance?

  2. Since the camera is tilted by an angle 13deg in the application ,is it necessary to do the calibration too with the camera tilted at respective angle?

mipe34
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dip
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3 Answers3

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My answer is "maybe" to the first question, and "no" to the second.

While it is true that it is not strictly necessary to calibrate with the target at the same or nearby distance as the subject, in practice it is possible only if you have enough depth of field (in particular, if you are focused at infinity), and use a fixed iris.

The reason is the Second Rule of Camera Calibration: "Thou shalt not touch the lens during or after calibration". In particular, you may not refocus nor change the f-stop, because both focusing and iris affect the nonlinear lens distortion and (albeit less so, depending on the lens) the field of view. Of course, you are completely free to change the exposure time, as it does not affect the lens geometry at all.

See also, for general comment, this other answer of mine.

Community
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Francesco Callari
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The answer is no to both questions. Camera calibration essentially finds the relationship between the focal length and the pixel plane when assuming the pinhole camera model; and optionally (as you will require due to your wide angle lens), radial distortion. These relationships are independent of the position of the camera in the world.

By the way, I see you tagged this as matlab: I can recommend the Camera Calibration Toolbox for MATLAB as a nice easy way of calibrating cameras. It guides you through process nicely.

devrobf
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That angle of the camera is not a problem, but you do want to calibrate your camera with the calibration target at roughly the working distance from it. In theory, the distance should not matter. In reality, though, you will have greater errors if you calibrate at 1 meter, and then try to measure things 5 meters away.

Also, please check out the CameraCalibrator App that is a part of the Computer Vision System Toolbox for MATLAB.

Dima
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  • Hi, I came accross multiple of your anwers on SE/SO when looking up stuff about camera calibration. I post my question here as a comment because I think it might not get a lot of attention. Camera calibration is something very specific. I read the answers you provided but unfortunately was unable to use this info in my situation. Would it be possible for you to share your insights? https://stackoverflow.com/questions/46225943/how-to-correctly-calibrate-my-camera-with-a-wide-angle-lens-using-opencv – privetDruzia Sep 14 '17 at 20:57