Vertex Distance Nuttiness!

[OptiStudent]

Supposing the need to compensate for vertex distance is warrented--What would I say (in the real world, not opticianry OptiStudent world) when I actually phone and order the lenses? Let me explain.
Data:
Rx: O.D. + 12.50 + 3.00 x 180
O.S. + 13.50 + 2.50 x 90
Add + 3.25
refracted V.D. 14 mm
fitting V.D. 9 mm.

Ok, the compensated power = +13.33 +3.47 x180. What do I order or answer???????????

Do I tell the lab the compensated power and let them round to dioptric steps or do I round myself prior to calling? If I round what is the correct procedure?

Secondly, (for Boards), should I answer with an answer rounded to dioptric steps? If so, again, what is the correct procedure? I have been taught 2 different ways that can give 2 different cylinder powers. Let me explain. For the OD:

1st Method: +13.33+3.47x180 is rounded before it's put on the optical cross to +13.37 @ 180 and +16.75 @ 090
= +13.37+3.37x180

2nd Method: +13.33+3.47x180 put on the cross as +13.33 @ 180 and +16.80 @ 090 and is rounded now to
= +13.37+3.50x180

I am confused, again. Hopefully, I wrote a followable post! I think the second method is correct. Thanks ya'll!

[Darryl Meister]

I wouldn't round. The values of the original Rx have already been rounded, and there's a good chance (50%, actually) that you would be rounding in the same direction, which would just make the prescription that much farther off. And I would use your first method for rounding powers; the cylinder should be the difference of the rounded powers from the optical cross (e.g., +3.375).

[Dave Nelson]

I hope I have understood your question correctly. If you ever fit contact lenses, you will run into the same problem over and over, since you are doing vertex adjustments and rounding off due to lens parameter availability. For myopes, I usually round up for those under 40, since they can accomodate any extra minus anyway, and down for presbyopes, who may find any added minus power uncomfortable, and using some of their already limited accommodative reserve. For cylinders, it hardly makes a difference, unless both the sphere and cylinder come out to amounts different from the .25 diopter increment, then you can round one up and one down to maintain the circle of least confusion. When ordering from your lab, you should specify the parameters that you want, and not leave it to them to figure it out. Most labs will also provide lens parameters in strengths other than .25 increments if you specify it, although I assume they would include an extra charge for machining a tool for you.

[EyeFitWell]

OptiStudent,
I'm in school right now also and my teacher explained that when she says "what will you order from the lab," that's our cue to give her a power that actually exists (and is rounded) versus the exact answer which is not rounded to eigth or quarter dioptors. You may want to double check with your instructor, but that's what my teacher said to look for on the exam.

Your answer will always be better when you use as exact of figures as possible up until the final result (in other words, method 2). As for rounding, it is the same rule for rounding any other number. If it's closer to the notch above, round up. If it's closer to the notch below, round down. Your textbook/teacher/exam should specify whether you're looking for quarter dioptors or eigth dioptors.

Good luck!