PD for FT28 with Considerable Prism

[NCspecs]

I'm about to order uncuts for a pt who has a Rx that looks like this:

OD Plano -1.50 X 100 ADD +2.25 4.00U
OS Plano -1.75 X 090 ADD +2.25 4.00D

Pt will be wearing Hoya FT28 iQ in Trivex (pt was previously in a standard Poly Ft28)

My question is this; I measured the pt's PD and found it, binocularly, to be 68/63 (distance/near). However I found his PD monocularly to be:

PD 68.5 33.0/35.5 (distance)
PD 63.5 30.0/33.5 (near)

Should I cut his lenses based on the monocular or binocular? I know it's not a significant difference and if he didn't have the prism correction I wouldn't give it a second thought. I just want to make sure I edge as accurately as possible according to his Rx. I did not have a whole lot of previous "hands on" experience with Prism until I started working for my current ODs. Mostly what I previously knew was from my own research and reading.

Thanks in advance!

[Barry Santini]

You must factor in his existing ft pd

I'd make mono nv:
Rt 32.0
Left 33.5

Btw, he'll luv his FF FTs in trivex!
Ex-3, i hope!

B

[NCspecs]

You must factor in his existing ft pd

I'd make mono nv:
Rt 32.0
Left 33.5

Btw, he'll luv his FF FTs in trivex!
Ex-3, i hope!

B

I think I will use EX3, but man, the turn around time has been rough during the holiday rush. my lab outsources so I usually get stuff back right at the ten day mark. I love the FT28 iQ- I've been converting all my FT wearers when I can. It's a good lens at a more than decent price. No reason not to use it.

So if I understand correctly, order and edge the lenses with the near PD 32/33.5? Thanks, this is exactly the info I need.

[Barry Santini]

Yes
I think this is the best pd for seg field overlap!

[marktheeyeguy]

If your patient has been wearing his current eyeglasses for any length of time. You should always, always take the PD off the current pair he is wearing. In addition, I would clock his old lenses and match the base curve. I would look at the seg ht he is currently wearing and mark the new one to fall in exact place of the old one. I will just say it like this, the old glasses are the road map to the treasure chest. What is correct isn't always right for the patient. I see it all the time. Optician makes new glasses patient can't see and all he says is its the patients fault there is nothing wrong with the glasses and he insist they are correct. Yes, they are correct. That doesn't mean they are right! I remake them matching up their old base curve, pd, sg ht, face form, panto, retro, and they love them! It's really simple. Stick to the basics. Remember, the old glasses are your friend. Nomatter as long as they have been wearing them for a period of time. Their eye has adapted to whatever base curve, pd, sg ht, adjustment they have been wearing. Match.it! You will have good success! Hope that helps! :)

[Barry Santini]

If you are edging these FT, be careful about slippage with EX-3. Also, you must know the "below" (mm below the datum line) of the seg. After a Trial blocking, check seg line to datum line distance, and confirm that it is correct and the same for both segments with this much vertical prism. You may have to alter the blocking value to obtain symmetrical and correct height for segs.

B

[Mick]

A good guideline for the blocking of prism lenses is to multiply the prism by .3, then adjust the blocking by this amount. Base down prism, raise the seg by this amount, likewise, base up lower the seg. Ex: 4 d prism up = lower the seg by 1.2 mm. Also base in prism you would move the seg (or oc) out by this amount. Base out move the seg in.

[PRECISIONLAB]

As Barry mentioned..caution when blocking. Parallax from prism may result in uneven seg heights from 1.0 to 1.5 mm off.

[NCspecs]

If your patient has been wearing his current eyeglasses for any length of time. You should always, always take the PD off the current pair he is wearing. In addition, I would clock his old lenses and match the base curve. ..... Their eye has adapted to whatever base curve, pd, sg ht, adjustment they have been wearing. Match.it! You will have good success! Hope that helps! :)

I agree completely with this advice and I strive to match the PDs, segs, and sometimes BC's, however you are unable to specify BCs with freeform technology. Regardless, I still think the Hoya is a good choice for this patient.

[Barry Santini]

These days, much less interested in the supposed importance of matching base curves.
IMHO, an rule of thumb now obsoleted by good FF.

B

[Robert Martellaro]

I'm about to order uncuts for a pt who has a Rx that looks like this:

OD Plano -1.50 X 100 ADD +2.25 4.00U
OS Plano -1.75 X 090 ADD +2.25 4.00D

Pt will be wearing Hoya FT28 iQ in Trivex (pt was previously in a standard Poly Ft28)

My question is this; I measured the pt's PD and found it, binocularly, to be 68/63 (distance/near). However I found his PD monocularly to be:

PD 68.5 33.0/35.5 (distance)
PD 63.5 30.0/33.5 (near)

Should I cut his lenses based on the monocular or binocular? I know it's not a significant difference and if he didn't have the prism correction I wouldn't give it a second thought. I just want to make sure I edge as accurately as possible according to his Rx. I did not have a whole lot of previous "hands on" experience with Prism until I started working for my current ODs. Mostly what I previously knew was from my own research and reading.

Thanks in advance!

Assuming that the +2.25 add is for 40cm, the near PD, using triangulation, will be OD 31, and OS 33.5 (.94 x 33 and 35.5 repectively). If the client is concerned about cosmesis, one might even out the inset to give 32.5 in both eyes.

Because of the vertical prism, our eyes will not point straight ahead. Instead, the right eye will turn downwards about 1mm, the left will point up about 1mm. Adjust the segs heights accordingly for the best function, compromising as needed for cosmesis (not recommended).

If your patient has been wearing his current eyeglasses for any length of time. You should always, always take the PD off the current pair he is wearing. In addition, I would clock his old lenses and match the base curve.

A lot of times I'll just start from scratch, noting the history, and except for segs heights (why are they always so low?!), I'll pretty much do it by the book. In most cases, it seems as if the old eyeglasses were not sorted out very well anyways.

[sharpstick777]

Although Free-form will be great for this patient, you would do well to advise him that his adaptation could be 20 minutes, and could be 2 weeks. You will be sending a lot of new math to the brain. He should do very well, in the end.

[jefe]

If you are edging these FT, be careful about slippage with EX-3. Also, you must know the "below" (mm below the datum line) of the seg. After a Trial blocking, check seg line to datum line distance, and confirm that it is correct and the same for both segments with this much vertical prism. You may have to alter the blocking value to obtain symmetrical and correct height for segs.

B

Or let the lab do it for minimal cost.

[NCspecs]

Or let the lab do it for minimal cost.

Believe me that would be my first choice but the Dr promised this pt in the room that he could keep his frames and that I could cut them while he waits.

[idispense]

Could someone post a diagram to illustrate the FT field overlap and to illustrate the triangulation method of pd calaculation. It might help for others to visualize the issues involved .

[Robert Martellaro]

The base of the triangle spans the eye's center of rotation, with the apex at the near object.

The formula for the near PD is...

NPD = DPD - DPD/1+ W(1/s - F/1000)

W is the work distance in mm. S is the stop distance (the distance from the
eye's center of rotation to the corneal plane, plus the vertex distance), and F is the lens power.

This does not take into account prismatic induced deviation due to horizontal distance power.

[idispense]

Thank you Robert, but now for the intimidated on here could you plug the individual numbers into that formula and show the entire calculation how you arrived atthe net answer, and if a diagram is available then we will have accomplished a great learning experience. Many may not know whathe stop distance value is . A complete explanation from step 1 and a visualization picture is worth a thousand words.

[idispense]

I would bet that many are wondering , "why not just use the NPD from the digital scope readings?"

[Robert Martellaro]

Thank you Robert, but now for the intimidated on here could you plug the individual numbers into that formula and show the entire calculation how you arrived atthe net answer, and if a diagram is available then we will have accomplished a great learning experience. Many may not know whathe stop distance value is . A complete explanation from step 1 and a visualization picture is worth a thousand words.

Using the right eye data.

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

I can't draw worth a darn (how 'bout you?), and I don't like math all that much (you had better check my work - you know this stuff better than I do).

I do like shortcuts though.

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

[idispense]

There is a
more detail on this subject that was posted earlier in 2008 see:

http://www.optiboard.com/forums/show...D-relationship

[Robert Martellaro]

I would bet that many are wondering , "why not just use the NPD from the digital scope readings?"

Two reasons- influence from frame position on the nose (same for the distance), and due to distance power on the horizontal meridian.

I found graphics that show teh near PD by triangulation. Thanks Harry.

http://www.optiboard.com/forums/show...l=1#post242804

CTR= center of rotation. Usually abbreviated as CR.

[idispense]

Thank you Robert, a picture is worth a thousand words, and thank you Harry, excellent as always.

[Robert Martellaro]

Thank you Robert, a picture is worth a thousand words, and thank you Harry, excellent as always.

Your welcome. Sorry I was a tad short earlier- free time is at a premium lately.

Harry's graphic is more informative than what I found in the "System for Ophthalmic Dispensing", which is usually pretty hard to beat.

To be sure, knowing how to achieve spot on accuracy for NPDs probably has more academic than practical significance, you never know when this might come in handy, although understanding how much and what direction to modify the inset due to distance power has more practical value, especially with PALs, with or without an unusually wide or narrow DPD.