# Camera pose using SolvePnP

**URL:** <https://forum.opencv.org/t/camera-pose-using-solvepnp/19476>\
**Category:** Python\
**Tags:** calib3d\
**Created:** [December 9, 2024, 1:01pm UTC](https://forum.opencv.org/t/camera-pose-using-solvepnp/19476 "2024-12-09T13:01:43Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![SimPer](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/simper/32/11428_2.png) [@SimPer](https://forum.opencv.org/u/SimPer)\
**Post date:** [December 9, 2024, 1:01pm UTC](https://forum.opencv.org/t/camera-pose-using-solvepnp/19476/1 "2024-12-09T13:01:43Z")

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Hi,

I am trying to determine the position of a camera with respect to certain marker points on a certain reference plate. I firstly calculate intrinsic parameters and the use those with solvePnP to calculate extrinsic parameters as well as the camera pose with respect to those markers. What I do not understand is, to what physical point does this camera pose correspond to. I read somewhere that this vector then points to the center of image plane, but I believe that it corresponds to the center of the entrance pupil.

Am I correct? Or is it some other physical point in the camera.

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**Author:** ![Steve\_in\_Denver](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@Steve\_in\_Denver](https://forum.opencv.org/u/Steve_in_Denver)\
**Post date:** [December 10, 2024, 7:10pm UTC](https://forum.opencv.org/t/camera-pose-using-solvepnp/19476/2 "2024-12-10T19:10:12Z")

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I believe the origin of the camera coordinate system corresponds to the pinhole (from the pinhole model). Since your physical camera uses optics (it’s not a true pinhole) the origin (pinhole) doesn’t correspond to some physical feature on your camera. I don’t think you can identify something (like the center of the aperture, or whatever) inside if the lens that will correspond to the camera origin.

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**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [December 10, 2024, 8:24pm UTC](https://forum.opencv.org/t/camera-pose-using-solvepnp/19476/3 "2024-12-10T20:24:59Z")

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what he said.

if your camera has _one lens_, then _its_ optical center is the origin of the camera. typically, cameras (even webcams) have more than one lens in their optics, and if the focus is adjustable, that point will move along with it.

which is why you calibrate a real camera to one fixed focus and zoom, i.e. adjust the lens system, then leave it, and calibrate that.

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**Author:** ![SimPer](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/simper/32/11428_2.png) [@SimPer](https://forum.opencv.org/u/SimPer)\
**Post date:** [December 11, 2024, 7:59am UTC](https://forum.opencv.org/t/camera-pose-using-solvepnp/19476/4 "2024-12-11T07:59:58Z")

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Thank you both for replying 🙂 As I understand it now, the camera origin is located in the optical center and even though it doesn’t match any other physical feature in the camera, it’s physical location can be related to the image plane as the center of image plane is translated for a vector of [0,0,f] with respect to the optical center in camera coordinates.