opencv 5.0.0; assumed calibration 0.005 mm/px; F = 29420 N, D = 10 mm
true d1/d2 (px) | measured d1/d2 (px) | d true -> measured (mm) | HBW true -> measured | in range
800/800 | 799.5/799.5 | 4.0000 -> 3.9975 (-0.06%) | 228.8 -> 229.1 (+0.13%) | True
800/780 | 799.6/779.5 | 3.9500 -> 3.9478 (-0.06%) | 234.9 -> 235.2 (+0.12%) | True
500/500 | 499.9/499.8 | 2.5000 -> 2.4993 (-0.03%) | 601.6 -> 601.9 (+0.05%) | True
1000/990 | 999.4/989.4 | 4.9750 -> 4.9719 (-0.06%) | 144.1 -> 144.3 (+0.14%) | True
400/400 | 399.9/399.9 | 2.0000 -> 1.9995 (-0.03%) | 945.5 -> 946.1 (+0.06%) | False

uneven illumination (linear brightness gradient across the image, grey levels):
  gradient   0: d = 3.9975 mm (-0.06%)
  gradient  40: d = 3.9978 mm (-0.05%)
  gradient  80: d = 3.9978 mm (-0.06%)
  gradient 120: rejected (segmentation not plausible: contour touches the image border; contour fills only 84% of the fitted ellipse)