Originally Posted by
Robert Martellaro
For every 1mm diference between the PD of the fitter and the subject, the fitter will over-estimate
the subject's PD by 1/16 mm if the fitters PD is larger than that of the subject. -Probably Jalie.
If your binocular IPD is 74mm and the subject is 58mm, and you don't compensate, your result will be off 1mm on the high side. Not usually an issue, but when it is, the compensation is typically .5mm to 1mm, maybe a little more if the subject is a very young child.
Keep in mind that these errors are additive, if the lab is 1mm wide on each lens, and your measurement method is 1mm wide per lens, and the lens power converges the eyes 1mm more per eye at the near point than the lens design anticipates, and/or if there is horizontal prism imbalance (base out in this example), we'll be looking at errors of 2mm to 4mm or more wide per eye! Admittedly that's the worse case but you get the idea- this can go very bad really fast if we're not paying attention.
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If you're not using a pupilometer, Use this chart to determine the near PD if the work distance is less than 16"/40cm.
If stop distance equals 27mm, the near multiplier for a work distance of
40cm is .937
35cm is .928
30cm is .925
25cm is .903
20cm is .881
Robert Martellaro
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