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Hello, I am in a fog right now...I’m taking a practice test for my license and it’s asking for the intermediate pd measurement....I know the reading is 1 1/2 off of monocular pd and 3 off binocular but what is the rule for intermediate? I’d appreciate any answers!
Quote:
Originally Posted by Bcandy1029
Best thing is use a PD meter, for near and intermediate, the rule of 3 mm off is not always accurate.
As a practical matter, I never worry about it. If you think about it, people wearing SV glasses never look through a near p.d., ever!
We only use near p.d. on SVNO because we can. They converge optics on PALs since they're so skinny. We inset segments because we don't like differential prism AND differential power.
What's more, if you provide it to a lab (instead of the distance p.d.) on a task lens it will goof them up.
There is a complex formula Darryl once posted, to get the actual number. You won't like it. But for spitballin' how about half the difference between D and N p.d.? But what defines "intermediate" anyway? Is it really "intermediate between infinity and 40 cm?....?
I have no idea what the official rule is- but as a general practice, we do a 3mm difference on PDs between 58-70 while adjusting 2mm for anything under and 4mm for any over. That is adjusting for near; I would still cut that for intermediate by half, rounding up.
Us spectacle wearing myopes are accustomed to BI prism when we perform near tasks- one might not want to use the near IPD when designing task lenses. These same folks might have unexpected discomfort when performing sustained near tasks after cataract surgery, depending on the final Rx of course. I usually use the distance IPD for myopes and full decentration (or more for segmented multifocals) for hyperopes.Quote:
Originally Posted by drk
Trifocals are usually half of the add, although most opticians today won't use this style of multifocal unless they work with low vision or have an elderly clientele.
Here's the formula for calculating the near IPD.
NPD = DPD - DPD/1 + W(1/s - f/1000)
W is work distance in mm
s is the stop distance (average 27mm)
F is the focal power
Quick reference for shorter work distances
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
Best regards,
Robert Martellaro
Robert,
You need to add a set of parenthesis to make this look right. NPD = DPD - DPD/(1 + W(1/s - f/1000))
Sorry guys, I do not understand: DPD-DPD is not 0?Quote:
Originally Posted by Kwill212
That's why the extra parenthesis are needed. It's hard to write nice looking equations on optiboard.
The second DPD is the numerator and the (1+w.....) is the denominator. This is the inset. Subtract that from the first DPD to get NPD.
No kidding. Not to mention my math and notation has been sloppy since the eighties.Quote:
Originally Posted by Kwill212
Here it is with values entered to make it easier to follow (and grin and bear my notation).
Mono PD 33mm and +2.25 add.
NPD = 33 - 33 / 1 + 400 x (1 / 27 - 2.25 /1000)
NPD = 33 - 33 / 1 + 400 x (.037 - .002)
NPD = 33 - 33 / 1 + 400 x .035
NPD = 33 - 33 / 1 + 14
NPD = 33 - 2.2
NPD = 30.8
And thanks for the hand written formula (you beat me to it), I think from one of the System for Ophthalmic Dispensing editions.
Best regards,
Robert Martellaro
I used a TeX editor to make the equation that can than be saved to an image file. http://atomurl.net/math/
Crystal clear now thank to all