Comment on this Rule of Thumb

3678 Measuring
THUMBJUMP DISTANCE ESTIMATION
Extend your arm in front of you and stick your thumb up. Place your thumb on the distant object you want to estimate the distance to; close one eye. Now close the other eye and open that one. Your thumb will seem to jump away from the object. Estimate the length of the jump over there. Multiply by 10.
Ken Harrison, retired scientist, alabama

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Ken Harrison - 2008-02-07
10 gives a pretty good answer for most. To refine it measure the distance from your eye to your extended thumb (not the same as your shirt size). Next get your PD (distance between your pupils). Divide the 1st by the 2nd, making sure they're both in inches or both in mm.