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Medical Forum / General / Vision / April 2006Trifocal segment powers & distances
I've never seen the diopters of power for the middle segment written on any of my trifocal prescriptions. So I'm assuming that the middle segment power is derived from the upper and lower segment power somehow. My current prescription is +1.50 sphere, with a 2.25 add. I'm guessing that the lower segment would be 3.75 diopters. But what would the power of the middle segment be if it's not written on the prescription? Also, is there a "typical" in focus distance they're trying to achieve with each of the 2 lower segments, and if so, what is the typical distance for each? Typical intermediate add is 1/2 the near add, or in your case +1.125 add or total +2.875. However, some tri's are avail. with different intermediate adds of 40%, 60% of the total add. I rarely need them, but have used them. w.stacy, o.d. > I've never seen the diopters of power for the middle segment written > on any of my trifocal prescriptions. So I'm assuming that the middle [quoted text clipped - 9 lines] > with each of the 2 lower segments, and if so, what is the typical > distance for each? +2.625 just in case anybody is paying attention... > Typical intermediate add is 1/2 the near add, or in your case +1.125 add > or total +2.875. However, some tri's are avail. with different [quoted text clipped - 16 lines] >> with each of the 2 lower segments, and if so, what is the typical >> distance for each? Thanks. Is there a "typical" in focus distance they're trying to achieve with each of the 2 lower segments, and if so, what is the typical distance for each? In other words, is there a certain number of inches away you should be from an object when viewing it through the reading segment and about how far for the middle segment? Obviously there are cases where you might want to deviate from that, but I just wondered if there was a rule of thumb involved. >+2.625 just in case anybody is paying attention... > [quoted text clipped - 18 lines] >>> with each of the 2 lower segments, and if so, what is the typical >>> distance for each? > Is there a "typical" in focus distance they're trying to achieve with > each of the 2 lower segments, and if so, what is the typical distance > for each? The "typical" distance has to be tempered for each individual. If you have to pick one, we might assume 18". If the near add is set for 18", the intermediate add should be clearest at double that (50% power) i.e. 36". -MT Doesn't have much to do with this but the Tangent Streak trifocal RGP contacts allow one to specify the add in the center segment independently (with the restriction that each segment has to be at least .75 difference). -Quick > Typical intermediate add is 1/2 the near add, or in your > case +1.125 add or total +2.875. However, some tri's [quoted text clipped - 16 lines] >> trying to achieve with each of the 2 lower segments, and >> if so, what is the typical distance for each? > I've never seen the diopters of power for the middle segment written > on any of my trifocal prescriptions. So I'm assuming that the middle [quoted text clipped - 9 lines] > with each of the 2 lower segments, and if so, what is the typical > distance for each? The 'typical' distance, as Mike Tyner said, is different for each individual. I always say to my presbyopic patients that the most important single question in their eye test is, "where do you want to be able to hold things to read?". The preferred reading posture leads to the power of the reading segment. There are guidelines and averages but everyone is different. Once you work out the power for the lower segment, the middle segment just 'takes care of itself'. Dom |
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