I'm talking with another OD who advocates excess decentration of segments to create BI prism at near only.
If you don't manage the distance optical centers, you will induce prism in the distance portion as well, I believe.
Question: Can you keep prism at zero in the distance portion while excessively decentering the bifocal seg?
You could do two things. Either calculate how much prism would be induced by the excess decentration and order the opposite or specify dist/near pd and the lab will do it for you.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Thanks, Wes.
I'm still not getting it.
On a semi-finished BF blank, say, plano with +2.00 add, there is an optical center of the seg, but no optical center for the distance portion, right?
Then, you add power to the back surface and you can, I assume, put the zero prism anywhere you want (by lens layout and blocking, I guess?). Right?
You would need to have the distance pd set before it is surfaced. Say for example you have a pd of 63/60 but you wanted to move the near in 4 mm to produce X amount of prism. You would ask the lab to grind a lens with 63/56. Once the lens has been surfaced you cannot move the bifocal without moving the distance.Originally Posted by drk
You want to drive your self crazy? Start spotting them all and measure the PD.
Segment placement and distance OC can be positioned as needed, independent of each other, limited primarily by blank size and edge thickness availabilty. I routinely ask for extra seg decentration to minimize, during the near gaze, induced base out prism on plus RXs.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
It's best to use a ft45 when decentering segs for prism.
Execs are the best to use. Franklins aren't hard to make. If all you want is a small amount of prism for anti-fatigue check with IOT or Signet Armourlite.
Yes. However, the two centers are not completely independent - you have to take the prism induced in the distance into account at the point on the seg through which the patient's near gaze will pass, as well as the prism induced by over-decentration of the seg.
Easiest way to calculate it, 2 step process...
1. Determine the amount of extra decentration you're going to use at the near (ie 6mm)
2. Determine what 6mm of extra decentration would do at the distance and order the opposite prism in the distance.
To say it all more simply, just increase the seg inset.
That means that part of the seg is (probably) going to disappear on the nasal side, reducing your ft28 to a ft 26 or 22 or.....
CME4SPECS has it right, use a 35 or 45 so you still have a normal size seg left.
THAT'S what was bugging me, Robert Shanbaum.
I figure that the patient is getting additional distance portion prism from looking more nasally than normally, and it's not so simple as decentering segs to achieve prism.
For example, take a SV lens power -3.00 and converge "3mm", and the patient gets an unintended ~1 pd BI. Now assume it's a BF -3.00 with a +3.00 add. If they align their eyes so they look through the segment's OC, then the total prism when reading is 1 BI from the distance portion + 0 from the near portion = 1 BI.
Now suppose the above patient aligns their eyes so that they look through another part of the seg, and not the seg's optical center. Suppose they converge 1mm past the seg's OC to 4mm. The prism from the distance portion is now ~1.2 BI and the prism from the seg is now ~0.3 BI so they get a net of 1.5 BI.
Suppose they converge only 1 mm. The prism from the distance portion is 1/3 BI, and from the seg 2/3 BO for a net of 0.5 BO.
In other words, they are always getting variable prismatic effect across the segment, depending on which section of the seg they look through.
The general case, then is this:
1. In all SV lenses, patients get variable amounts of prism when they converge to read. If the patient is high minus -9.00 and they converge "3mm", they'd be getting whole lotta BI prism, so they'd converge less to find their own comfortable position.
The better way to state it, instead, is that convergence is variable, depending how much lens power there is and how much prism the eye muscles prefer. No two will be exactly alike. (Individualized progressive lens manufacturers know this and hence variable insets by add power and distance power.)
2. By slapping a segment on top of the "variable prism" SV lens, we get an additive or subtractive "variable prism". Depending on inset, distance portion power, near portion power, etc. you can get some odd combinations.
3. With variable prism effect, the eyes can't really be "forced" to look through any particular portion, so you really don't know what you're delivering (assuming you're a stickler for such details and that it matters). We see this same issue with prescribing PALs to deliver plus power to an accommodative/esotrope kid: How much plus does the kid really get because though what portion of the near zone/corridor is she really looking? (Yeah, short corridors are best for this.)
4. The issue isn't really where the distance OC is placed to compensate...it's that the "lens" prisms are variable in magnitude.
So, I go back to Jacqui's post: From a purely optical perspective, aren't Franklin (non-variable prism) segs superior?
Last edited by drk; 08-27-2011 at 08:58 AM.
Franklins have O.C.s too.
Hmmm...
Yes, but am I wrong? You can surface the lower portion with prism, so it's "inescapable" and non-variable.
Nonetheless, you have a point; if the power is non-zero in the lower portion, there will be an additional component of variable prism introduced in this method, as well.
Tell your friend him not prescribing opposite prism in the distance to counteract the induced prism is switching one problem for another in high myope cases it's even worse since decentration in the distance will induce greater prism, in the case of low myopes -2.50 with a 2.50 add it's making everything worse since that particular rx can only be fixed by a Franklin.
Good luck in your argument.
If you manufacture the franklin yourself from 2 lenses you can put the o's anywhere you want and totally eliminate the prism. Everything else will have some tradeoff.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
I'll try this again because it seems to me that you're answering a different question, or I'm missing something.
The lab does this more or less automatically. If you order 65/58, that's what you'll get. If you want the distance OC 5mm above or below the seg in addition to the above, well that's doable too. You can have your cake...If you don't manage the distance optical centers, you will induce prism in the distance portion as well, I believe.
The answer is yes. For example, you determine that your subject wearing +2.00 sph Add +2.50 @40cm needs 1^ total BI at near only. The new Rx is essentially the same as old. PD is 63/59. Order 63/55.Question: Can you keep prism at zero in the distance portion while excessively decentering the bifocal seg?
My conclusion is that your colleague's statement is correct. It better be, because I've been doing this for thirty eight years.
PS
Now if your asking if you can ignore the prism induced as the eyes converge behind the distance lens, the answer is... it depends. I don't refract so bear with me now, but it depends on how you determine the need for prism at near. If it's done with handhelds over the eyeglasses with the eyes in the inferior gaze, it can be ignored (if the Rx change is minimal).
However, if you refract for prism with the full reading power in place in the phoropter, you'll get too much BI prism because you're including the near power in the prism calc, which does not equal the prism behind the distance Rx in the eyeglasses. Using the above example, the prism induced behind the distance Rx at near is .4^ BO per eye. If you include the Add power, it's .9^ BO per eye. If the result for prism tests out at 2^ BI total, you need to subtract 1^ (.9^ BO -.4^BO x 2 = 1^ BO) for a total of 1^ BI.
If you set the phoropter for the near PD, then increase the result by .8^ BI (to compensate for the eyeglasses induced BO prism at near).
PPS
Welcome back!!!
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
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