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This is a bracelet I need the size of the pearl

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    There has to be an object in the image with known dimensions.
    – Wolff
    Dec 1, 2023 at 21:26
  • Three identical photos are not more useful than one. If they were shot from different directions with the same camera focal length some photogrammetric reasoning might be possible. The same reasoning would be possible as well if the pearls were identical i.e. you had it seen from different directions in the same image. But the focal length of the camera lens is a must for such calculations. Your photo is anonymized, there's no metadata of camera settings. Dec 1, 2023 at 22:36
  • (continued) The remaining way is to know the width of one shown parts and calculate the pearl size proportionally from the width as pixels. Jewel making hobbyists and professionals may know just the shown metal parts and compare the pearls to them. They may know even the pearls. Note: every photo can be a counterfeit which needs an experienced analyzer to see it's a fake. Dec 1, 2023 at 22:44
  • @Abuabapplipbleable it's worse than that. The focal length alone can't tell you anything about the real size of an object in a photograph, you'd also need much more information: the sensor size, the distance from subject to camera, the image size of the original photograph in pixels (not a crop), and you'd need to measure the size of the object in pixels too. There's a formula for this somewhere I once saw on Photography SE. The only real answer here is you can't measure the size of a real object from a photo alone. It's not possible when there are so many unknown variables.
    – Billy Kerr
    Dec 2, 2023 at 12:37
  • Not true. Photogrammetry based point cloud generation algorithms search which 3D scene fits to a set of photos which are shot of the same scene from different directions and distances. The algorithm doesn't know the size of the pixel. It assumes the camera focus settings and the focal length are the same and it knows the focal length. The pearls are problematic here because there's very little details on them so it's difficult to find corresponding pixels between different pearls. The other pieces are easier, so a reference dimension maybe could generated of the metal parts. Dec 2, 2023 at 13:42

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