I have an image of hexagonal close-packed particles and I wish to find the distance between the centers of a particle and its closest neighbor particle. Using the manual counting method as explained here, I found that distance to be around 0.81um.

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**Plot profile**

Using FFT, I have an image with concentric rings about the centre. The radial distance of the innermost ring, which value should be the distance I wish to know, is 0.71um/cycle instead.

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**FFT image**

Why is there a discrepency? Am I missing something?

**Image file**