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GoPRO focal length in videos

asked 2016-07-28 05:04:42 -0500

ale_Xompi gravatar image

Hi all,

I would like to know if the focal length varies when recording videos. Because of auto-focus, I believe that for each recorded video the focal length is changing but how much this change can affect 3-D re-projections. Besides, I also don't know if the focal length remain fixed when recording or still it can change.

Please let me know something about that. Thanks.

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answered 2016-07-28 06:24:24 -0500

The GoPro cameras have a fixed wide-angle lens. You can change FoV / focal length of the sensor readout, to some degree. See this field of view table for the Hero3+ Black.

Because GoPro lens parameters, the hyperfocal distance is quite near. The EXIF tags of captured images say~ 0.5m. Since almost everything is in focus, the camera doesn't have to resort to autofocus.

Correspondingly, you don't have to take care of auto-focus issues with these cameras.

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So, this means that once you have calibrated a GoPRO camera (either Hero3+ or Hero4) and if you don't change settings (resolution and FOV), you can use the intrinsic parameters for all video recorded later. Am I correct?

ale_Xompi gravatar imageale_Xompi ( 2016-07-28 09:38:06 -0500 )edit

Quite. What stays constant per camera regardless of resolution and FOV is the _distortion coefficients_.

The other camera intrinsics will change per FOV/resolution setting. Depending on the parameters you choose cv::fisheye::calibrate and cv::fishey::stereoCalibrate will change your focal length to align left and right views to each other. Also the camera center is dependent on image resolution.

In you case: - Determine distortion coefficients for the widest FOV per camera. - Determine the other intrinsics per FOV/resolution setting, per camera with known distortion - stereo calibrate your each fov/resolution setting - use these happily ever after

lnz-ptng gravatar imagelnz-ptng ( 2016-07-30 06:05:04 -0500 )edit
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Asked: 2016-07-28 05:04:42 -0500

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Last updated: Jul 28 '16