# Focal length for di̇stance

**URL:** <https://forum.opencv.org/t/focal-length-for-di-stance/18309>\
**Category:** Python\
**Tags:** calib3d\
**Created:** [July 19, 2024, 10:04am UTC](https://forum.opencv.org/t/focal-length-for-di-stance/18309 "2024-07-19T10:04:45Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![akdenzi](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/akdenzi/32/8957_2.png) [@akdenzi](https://forum.opencv.org/u/akdenzi)\
**Post date:** [July 19, 2024, 10:04am UTC](https://forum.opencv.org/t/focal-length-for-di-stance/18309/1 "2024-07-19T10:04:45Z")

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Hello everyone,

I have a question regarding distance measurement with a camera. To measure distance from the camera, we need the focal length. From my research, I understand that we can determine the focal length by performing camera calibration. Additionally, the focal length is provided in millimeters in the technical specifications of the camera. Can we use this focal length value from the camera specifications to measure distance? Should we use it in millimeters? I would appreciate your guidance on this matter.

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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:** [July 19, 2024, 2:33pm UTC](https://forum.opencv.org/t/focal-length-for-di-stance/18309/2 "2024-07-19T14:33:29Z")

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> <https://stackoverflow.com/questions/69159247/camera-calibration-focal-length-value-seems-too-large/69161589#69161589>

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<div class="post-metadata">

**Author:** ![akdenzi](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/akdenzi/32/8957_2.png) [@akdenzi](https://forum.opencv.org/u/akdenzi)\
**Post date:** [July 19, 2024, 2:58pm UTC](https://forum.opencv.org/t/focal-length-for-di-stance/18309/3 "2024-07-19T14:58:06Z")

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thanks.So, is the focal length of 25mm written in the camera’s technical documentation sufficient for me? Do I need to calibrate the camera?