How can I determine a smartphone camera's focal length in pixels for pose estimation?

Asked 9/13/2012

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I’m doing camera pose estimation and need my smartphone camera’s focal length expressed in pixels. Is there a reliable way to get this for a phone camera, rather than estimating it from the advertised focal length?

opencv pose-estimation smartphone focal-length camera-calibration

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

13y ago

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This is really easy. Matlab camera calibration toolbox (Bouquet) or OpenCV camera calibration will do this for you if you take 10-20 images of a checkerboard. The intrinsic parameters give you the true center of the lens in pixels and the focal length in pixels. It will be pretty close to [focal length in mm]*[resolution]/[sensor size in mm], but due to mechanical inaccuracies not close enough for accurate pose estimation.

Originally by user11455. Source · Licensed CC BY-SA 4.0

user11455

13y ago

AI Answer

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For pose estimation, the reliable way is to calibrate the camera and recover its intrinsic parameters. Use a calibration tool such as the Matlab Camera Calibration Toolbox or OpenCV camera calibration, and capture about 10–20 images of a checkerboard pattern. The calibration results will include the focal length in pixels and the principal point in pixels.

A rough estimate can be made from:

focal length in pixels ≈ focal length in mm × image resolution ÷ sensor size in mm

But for accurate pose estimation, that estimate is usually not precise enough because of mechanical and manufacturing tolerances. Calibration is the better approach.

How can I determine a smartphone camera's focal length in pixels for pose estimation?

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