Why Is an Astigmatism Correctionly Only Half As Strong As a Spherical?
April 6th, 2010 | See the Top Answers to this Question Below
I hope that makes sense. I read that if you have -1.00 of astigmatism it’s the equivalent of -0.50 of spherical. Why is that?
Is it the same if the prescription is +1.00 of astigmatism? It’s quite complicated isn’t it?
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its not that complicated once you get the hang of it. there is no such thing as a plus cylinder/astigmatism correction in lenses. i cant remember off the top of my head why cylinder is still written in plus on ophthalmologist prescriptions. but if you have a plus cylinder, you add that to the sphere power. so a -.50 sph and a +1.00 cylinder would bring you to a +.50 sph.. but then you would change the cylinder to a minus.. and change the axis by 90 degrees. its called transposing.
Visually the two units have little to do with eachother unless you need to transpose. in short one controls the size, the other is fine tuning.
its the shape of your eye.
I don’t really understand your question…. My optician writes down both my eyes as the same kind of thing, but I have an astigmatism in one eye.
Astigmatism is an overall inability of the eye to focus clearly at any distance because of uneven curvatures of the cornea. Essentially, the cornea is oval, having a surface shaped more like a football or the back of a spoon rather than being rounded like a basketball. Very frequently associated with myopia, astigmatism can cause light rays to focus at more than one point on the retina, and objects at all distances appear distorted—much the same way as some fun house mirrors make images too tall, wide, thin, or short.
If you want detail, this is a helpful site.
http://www.1800contacts.com/staticcontent/vision101/frames.html