XPost: sci.med.vision   
   From: robopt@nospam.com   
      
   On Tue, 07 Feb 2012 13:41:32 GMT, liz  wrote:   
      
      
   >Thank you for the useful feedback Robert. I bought the bifocal specs to   
   >use when shopping, so they combine far vision with near vision (+1.50   
   >D).   
      
   The Add power is +1.50? Do you wear your glasses full time? Do you read or   
   perform general close tasks with the eyeglasses off?   
      
   >I get surprisingly dizzy in a supermarket even with single vision   
   >lenses, so we decided on a frame with a large eyepiece and the optician   
   >suggested a very low close up segment.  Both factors were chosen to   
   >minimise peripheral uncorrected vision.   
      
   Generally, smaller and closer (to the eye) minimizes off-axis aberrations and   
   distortion. The segment is usually level with the lower limbus or eyelid, but   
   might be set slightly lower in some instances, for instance, very little close   
   tasks, or when they are removed when reading.   
      
   >I don't know anything about yoked prisms but after your post I read they   
   >are used to "move the world in one direction".  Are you saying you can   
   >tell from my photo that I have got yoked prisms or are you saying that I   
   >should consider them?   
      
   The prism is induced by placing the vertical optical center at some other point   
   besides center pupil. I'm concerned that the unusually lower segment position   
   may have lowered the OC too far. The OC should be about 3mm t0 5mm below the   
   pupil. More than three diopters or so of yoked prism can cause some discomfort.   
      
   BTW, I got it wrong- the prism induced with your Rx, if the OC is below the   
   pupil, is base up, not base down.   
      
   If you can not wear the eyeglasses without feeling dizzy, even SV as you stated   
   above, and the lenses are optimally designed and positioned, one might ask the   
   doctor to reduce the astigmatic power somewhat. This may increase the   
   wearability, but will decrease the visual acuity slightly. This can be   
   demonstrated in the chair, or better still with a trial frame that will allow   
   you to walk around to better evaluate performance.   
      
   Robert Martellaro   
   ~~~~~~~~~~~~~~~~~~   
   Roberts Optical Ltd.   
   Wauwatosa Wi.   
   www.roberts-optical.com   
   ~~~~~~~~~~~~~~~~~~   
   "Science is a way of trying not to fool yourself."   
   - Richard Feynman   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)