Santini, santini,marone a ma mia, your still not getting it, i,m aware of what your allowed to be off, thats not the question.
The question was, is if the phoropter is off a couple of millimeters from what the patients PD is, will that throw off the possible needed prism, but now according to the doc,s the answer is no.
To simplify:
You have to account for positive and negative fusional vergence ranges...measured to blur/break/recovery. These ranges can be improved first with vision therapy. Second is prism. The amount of prism is not what is actually measured. Vertical fusional ranges are far less and do not benefit from VT.
Horizontal prism (unwanted and induced) would most likely be negligible in the horizontal ranges....vertical can cause more problems.
Another system that some here have not considered is the effects of accomodative convergence and accommodation (ac/a ratios). Someone with a high ac/a ratio will be more influenced by lens power.
Try not to over analyze just one aspect...look at the whole system...
It is, when you consider that the cumulative tolerances from fabrication, measuring, prescribing, and induced prism from the lens design can cause us trouble, especially with the eldery, where fusional reserves might be a fraction of younger, healthy eyes. My rule of thumb has been to try to miss wide on minus, and narrow for plus.Originally Posted by Barry Santini
But I wonder how the docs deal with a induced VI with anisometropes. For example, when you Rx 2^ BU OD for a client wearing Rt +3.00 D Lt +1.00 D in a 4 drop PAL, we need to know if you're asking for additional VI on top of the .8^ of induced VI, which would be typical if you placed a hand held over the eyeglasses, or 2^ total, using well centered lenses in a phropter or trial frame.
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.
Again I think you are getting too specific. Sure for fabrication, you need tolerences...but in the determination of the prism amount, its not an exact science like a chemical reaction..
Using your example, yes there is a vertical imbalance...but how was the 2prism diopters found? A hyper deviation may not be equal from distance to near, or in 9 cardinal directions of gaze.
A vertical prism bar is not in half diopter steps. An estimation is used usually. I recently observed a MD use only a 4 and 8 diopter lens to estimate a hyper deviation during a neurosensory exam. I tend to use 1 diopter prism increments. Again, the amount is usually reduced for a verical deviation.
Hi, speaking of prism, What is the difference between making the prism and descentering to induced prismatic effect.
\
Prism is prism, whether you get it by grinding/blocking it in, or decentering it in.
Ok, I agree wih you but, what happen if you have OD: +3.00 1 BO Dp
and in OS: +1.00 with no prism prescript. If you grind will no have trouble, but if you descenter you will get that when the patient looks ahead but if he turn to the right you will no get the same prism effect. what is your opinion about it.?
In your case stated, Lado, having the lenses surfaced is better, in order to balance thickness, control base curves, but theoretically the result is the same.
Prism is created by displacing the O.C.
Prism efficiency is, visually correct, and functional, at patient's direct line of sight(IPD). The amount created by excursion gaze is not the prism prescribed.
Anyone ever notice that you won't see prism prescribed in any direction other than up,down, in, or out twice in your working lifetime. Ever wonder why it's never expressed in odd degrees (and yes when it is rarely Rx'd in odd degrees there are 360 degrees not just 180 as in cylinder) ?
Chip
Funny you should mention this....and with St. Patty's Day just around the corner, as ye might be sayin'!!!
I worked with an Irish ophthalmologist who used to prescribe it in precisely this way.
Green Beer anyone?:cheers::cheers::cheers:
my thoughts are very simple. if the refracted pd does not match what we measure when fitting the glasses. it will always be a huge problem. if the ecp provides the PD to be used. ( which they almost never do) problem solved> sometimes without all of the correct information on an outside rx, we need to learn to pass up the sale.
Donald W Summers,B.S,A.B.O.C,N.C.L.E
Yes but what if you have prism on one train that leaves the station at 9:43 AM, and travels west at an average speed of 14.789462734 meters/sec., and you have a European Swallow traveling north with a coconut (de-husked of course to reduce coefficient of drag...DUH!) along the lines of magnetic polarity parallel to the prime meridian but offset by the standard 56.2 nautical miles to the west to avoid traffic from Heathrow. And then you have the obligatory submarine piloted by a one-eyed parrot ascending from the ocean floor and an initial pressure of two hundred atmospheres at a rate of 5.8932567823 meters/sec...divided by pi of course (come on people, do keep up here!)...
WHAT WOULD BE THE PRESCRIBED PRISM OF THE COCONUT THEN?!?!?!?!?!?!?!?! And I want STRICT adherence to ANSI Barry! ;)
:cheers::cheers::cheers::cheers::cheers:
Happy St Pats all!
The answer: Start with 1dp base in, 1dp base in, and 1dp base up.
A coconut has 3 eyes
I see a lot of prism Rxs (about one per week) and still see an occasional 360 notation. I filled an Rx a couple years ago that used 180 notation (OD 6^ BD x 30)!
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 all depends on how many days til vernal equinox.
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Reply With Quote
Bookmarks