After cycloplegia, a light source at infinity looks as a vertical line 1 diopter before the retina and as a horizontal line 0.5 diopter after the retina. What would be the refraction? what would be the prescription? I would appreciate it if you could also explain why. Thank!
as your being an Ophthalmologist, you should be able to teach us those things.
thanks for the constructive answer. I'm a resident before my boards.Originally Posted by Chris Ryser
One meridian is in front of the retina, resulting in myopia, requiring a diverging lens to bring the rays into focus on the retina, corrected with minus dioptric power, the other meridian comes to focus behind the retina, resulting in hyperopia, requiring a converging lens to focus the rays onto the retina, corrected with plus dioptric power.
Because there are two principle meridians this results in astigmatism. This example is called mixed astigmatism because one principle meridian is in front of the retina, the other is behind the retina. Astigmatism is corrected with a cylindrical lens.
The principle meridians are -1.00 x 90, and +0.50 x 180. The difference in power between the two meridians is the cylinder power, +1.50 x 90 or -1.50 x 180. Note: the cylinder power is at ninety degrees opposite of what is written. Go figure. We can demonstrate this better using an "optical cross". Maybe another Optiboarder can show this with a diagram, (and please check my work for accuracy).
The Rx can be written in plus cylinder form (+1.50) when using a phoropter with plus cylinder lenses as -1.00 +1.50 x 90, or in minus cylinder form (-1.50) when using a minus cylinder phoropter as +0.50 -1.50 x 180.
Hope this helps,
Robert Martellaro
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.
vox,are you refracting in cycloplegic mode?
Last edited by COMEINPEACE; 04-14-2017 at 03:32 PM.
Dr. Robert Martellaro don't need no stinkin' board certification!
JK,
I could squeak by the ophthalmic optics courses, but what's that, less than 5% of the optom curriculum, and even less for ophthos? Regardless, thanks for the kind words.
Voxexo,
I wrote the above with 1 foot out the door, hence the poor phraseology. I'll be away from Optiboard for a couple days, but if you have any questions there are plenty of others who'll help out. Just ask.
Best regards,
Robert Martellaro
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.
Thank you so much!
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