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Medical Forum / General / Vision / February 2008Thinnest point vs optical center - prism effect?
My prescription is: OD -4.50 -0.50 135, OS -1.75 -4.25 175, BIFOCAL ADD: OD +2.00 OS+2.00 (Half moon). There is no prism called for in the prescripton. The thinnest point of the right lens is 3 mm higher than the left. This was measured from the seg line. I think this is called "induced prism" or "prism by decentration". The optician maintains that the optical centers (OC) are correct and that there is no prism in the glasses. The optician also says that the OCs do not correspond to the thinnest point of the lens because they are bifocals. Is it possible that these glasses do not have prism? Regardless of what you measure, it's easy to identify. Put the glasses on and choose a small, distant object. Be sure you're looking straight ahead, through the centers of the lenses. Cover one eye with your hand, then quickly switch to the other eye, and repeat back-and-forth to see if the distant object jumps up or down. It might jump side-to-side; that isn't relevant. Vertical prism produces only up-and-down motion. If it doesn't jump up or down significantly, then either there is no vertical prism, or the prism is exactly right for you. If there is significant prism, it's usually hard to wear the glasses. -MT, OD > My prescription is: OD -4.50 -0.50 135, OS -1.75 -4.25 175, BIFOCAL ADD: > OD +2.00 OS+2.00 (Half moon). There is no prism called for in the [quoted text clipped - 10 lines] > > Is it possible that these glasses do not have prism? >Regardless of what you measure, it's easy to identify. > [quoted text clipped - 3 lines] >Cover one eye with your hand, then quickly switch to the other eye, and >repeat back-and-forth to see if the distant object jumps up or down. Now you've done it- the entire eyeglass wearing population of planet earth is going to start walking around covering one eye checking for VI. Ay caramba! >> My prescription is: OD -4.50 -0.50 135, OS -1.75 -4.25 175, BIFOCAL ADD: >> OD +2.00 OS+2.00 (Half moon). There is no prism called for in the [quoted text clipped - 3 lines] >> was measured from the seg line. I think this is called "induced prism" or >> "prism by decentration". Sounds like an engineer. Calipers are good for measuring thickness but not for prism. >> The optician maintains that the optical centers (OC) are correct and that >> there is no prism in the glasses. The optician also says that the OCs do >> not correspond to the thinnest point of the lens because they are >> bifocals. The vertical distance OCs can be placed anywhere from the top of the lens to the bottom of the bifocal, equal or unequal in height. Unequal heights would be appropriate if the eyes are not level. >> Is it possible that these glasses do not have prism? Yes, it's possible. At this power, the OCs should be + or - 1mm total on the vertical meridian, and + or - 2.5mm total on the horizontal meridian, (from the ANSI Z80.1-2005 standards) referenced to the specified position. I've specified monocular PDs with disparities of as much as 4mm, and vertical centers of 3mm. The optician should place a plus sign at the distance OC of each lens and verify that the lens aligns with the center of the pupil for each eye. Hope this helps, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman |
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