Post by Mike TynerIt might take two or three prism diopters horizontally to feel a difference.
Some folks can tolerate that much unwanted prism on the horizontal meridian. The
ANSI Z80.1-2010 Summary http://www.opticampus.com/tools/ansi.php
recommends no more than .67^ total for powers below ±3.37 D. That's achievable
with a reasonable amount of care and precision from the lab and optician.
We won't get into trouble if we follow these recommendations.
Post by Mike TynerWith a -1.00 rx, three prism diopters = 30 mm pd error.
It's pretty hard to make a client miserable at that power due to prismatic error
from improper centration, especially with SV lenses in front of young, healthy
eyes. However, aspherics require precision regardless of power, although it's
not likely we'll use aspherics for a -1.00. But it's not unusual for a ±3.00.
Post by Mike TynerUnilateral PDs are only important for progressive-addition lenses, and not
crucial even there.
Lens is a PAL. Mono Pd is 35/32. PD given to lab is 67, forcing the lab to use
33.5/33.5. Lab supplies 32.5/34.5, which is within Ansi's ±1mm per eye
Horizontal Fitting Point Location recommendation. This results in both lenses
being mis-positioned by at least (depending on the skills of the PD taker)
2.5mm. The PAL in question has a corridor width of 3mm at the .50DC limits. With
each eye tracking along the surface astigmatism boundary, client complains of
general discomfort at intermediate and near, with slightly reduced contrast and
acuity, reporting that the old lenses were better performing, unusual
considering the lenses were replaced due to scratches only- same PAL design,
material, frame, Rx, and seg height!
Post by Mike TynerUnilateral PDs double the opportunity for error.
But binocular PDs increase the chance for errors to 100% when the monocular
(unilateral) PDs are not equal.
Robert Martellaro
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Roberts Optical Ltd.
Wauwatosa Wi.
www.roberts-optical.com
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"Science is a way of trying not to fool yourself."
- Richard Feynman