Technical trivia questions of the day:

[DC Optix]

In your initial question, the angle is given. "..at a 45 degree angle up at the nasal." That (at least to me) means up and in which would be @45 for a OD lens or @135 on an OS lens.

The formulas I've given would tell you the amount of prism to order (in, out, up or down) if only the amount of prism and the angle (axis) is known.

To reverse this ( knowing the amount of vertical and horizontal prism requested) and find the amount of prism and at what angle (axis), you would use your resultant prism formula;
P² = H ² +V ² √

P² is the amount of resultant prism
“H” is the horizontal prism amount
“V” is the vertical amount
√ is square
To find the angle (axis) you would divide the vertical amount by the horizontal amount then Tan‾¹.

Example;

O.S. 5 out and 3 up

3 ÷ 5 = Tan‾¹

Your calculator should say 30.963…

So the final Rx checked on the O.S. Should be 5.8 diopters of prism @ axis 31. Keep in mind the direction of the requested prism in relation to which lens you are neutralizing. If the above prism amounts were on a right lens, an axis of 31 would be up and in. ( wrong direction!)

When you Tan‾¹ your prism you are only getting the angle from your 0-180 line. If your prism direction is down you would need to add (to 180) or subtract (from 360) that angle (using 360 notation) depending if it’s a right or left lens. Clear as mud?

24/7...THANK YOU SO MUCH!! That is EXACTLY what I was looking for! I don't know why I have such a hard time with this, but alas, I do! Thanks again!

[DC Optix]

Oops...hit "post" too soon! The rest of my post should've read:

Now, I know that accounts for the 7D @ 45. However, It doesn't appear to me that it accounts for the existing 6D base in, that the patient walked in wearing. So, just to be sure here, you would then add the 5 up and 5 in to the 6 in, resulting in 5 up and 11 in, right?

Thanks again, everyone!

[DC Optix]

Ok, so going back and piecing together all the posts from above, I think I got it all! Whew!