How come I got different results measuring the size of a repetitive pattern (e.g. picket fence, windows) in the same image
Small variation is inevitable. If variation is large, the most likely cause is that the camera’s lens has significant aberration. The telltale sign of lens aberration is that long vertical lines (tall book shells, doors, buildings, etc.) appear curved. Distortion is usually most pronounced at the edges of the image. So if you obtain inconsistent measurements of a repetitive pattern, you should trust those from the center of the image more.
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