1. ## Toric Vertex Compensation

Spectacle RX -4.25 -.75 X 090
Contact Lens RX -4.00 -.75 X 090

If X is the spherical and Y is the Cylinder what powers are they?

No cyl at 90 correct? Y would be -4.25
all in effect at 180. X would be -5.00 no?

Also, do you round up or down when simply splitting -1.75 at 45 or 135?

2. Originally Posted by skirk1975
Spectacle RX -4.25 -.75 X 090
Contact Lens RX -4.00 -.75 X 090

If X is the spherical and Y is the Cylinder what powers are they?

No cyl at 90 correct? Y would be -4.25
all in effect at 180. X would be -5.00 no?

Also, do you round up or down when simply splitting -1.75 at 45 or 135?
Your question isn't exactly clear since you have already both the Spectacle Rx and Contact lens vertex compensated Rx.

Prescribed RX - SPH power at 180 = -4.25D, total power at 090 = -5.00D

3. Originally Posted by skirk1975

Also, do you round up or down when simply splitting -1.75 at 45 or 135?
DRK would say (correctly) use the 1/8 diopter answer (0.87) in the vertex formula.

4. Originally Posted by Lab Insight
Prescribed RX - SPH power at 180 = -4.25D, total power at 090 = -5.00D
Maybe I'm missing something, but I believe it's just the opposite of this. (sph @90 -4.25, power at 180 -5.00)

5. If I read the question "If X is the spherical and Y is the Cylinder what powers are they?", then "X" (which is the sphere component AT the 90 degree meridian) = -4.25D which vertexes to -4.00 (assuming ~14mm spectacle vertex distance) at the ocular surface.

"Y" (which is the sphere component plus cylinder component AT the 180 degree meridian) = -5.00D which vertexes to about -4.62 at the ocular surface.

So, if I'm getting the question right, the exact contact lens would be: -4.00-0.62 x 090.

But the closest actually available contact lens will be : -4.00-0.75 x 090.

As to the second question, I'm not getting it. Can someone explain it to me?

6. Lab Insight- you done goofed.

24/7 - I don't think you are missing anything. Power at 180 is -5.00.

Drk - I believe they want to know of they should round the resultant cylinder power at 45/135 degrees of -0.875 up to -1.00 or down to -0.75. I think there is a typo and they mean -0.75 at 45/135 not -1.75, so rounding from -0.375 up to -.50 or down to -0.25. It all seem very strange for a contact lens question.

7. OK, I trust you.

As a general rule (almost an absolute law of the known universe) one always rounds down for astigmatic correction.

Otherwise, the orbits of the outer gas giants will be slightly altered.

Maybe I'm missing something, but I believe it's just the opposite of this. (sph @90 -4.25, power at 180 -5.00)

That’s what I thought too!!!

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