I know this has been discussed before-I should understand this by now, but this is confusing...I have an individual whose Rx is OD: -3.75-0.50x045, OS: -4.00-1.00x090! The FPD: 70 (51X19), PPD: 63, " B " size is 44mm.
Normally, decentration would be 3.5mm in...However, I noticed this persons eyes were ( after checking the frame for excessive wrap, proper face form etc. ) 4mm up from the Datum line and 4mm in towards the nose. I figured since I'm fitting this guy with a mid-index ( Spectralite 1.54 ) it's almost like peeking through a hole in a fence, i.e., this Rx has got to be right over the pupils!... My lab called me and said, " We can't decenter anything over 3mm because you ( meaning me as an Optician ) would be inducing unwarranted prism! Now my OD says, " Well, what difference does it make since the lenses are being surfaced with this Rx; your ( meaning me again ) just telling the lab where his pupils are exactly located ".... Alright so let's see, power in the right should be -4.00 ( since we are 45 degrees away from the 90th, that's 50% of cyl. ) and we are already at 90 degrees in the OS; that remains at -4.00... If we go 4mm up and 4mm in on each eye that's 8mms of movement in each eye... -4.00x.8= 3.20^BU...I believe when figuring vertical imbalance and the directions are the same these figures cancel each other... Ladies and Gentlemen.....your assistance please!
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