Astigmatism?

Started 11 months ago | Questions thread
alanr0 Senior Member • Posts: 2,073
Re: Astigmatism?
1

RSColo wrote:

OpticsEngineer wrote:

"Do glasses have corrections for multiple "wrinkles" in our eyes."

"Wrinkles" like that would be called high order optical aberrations. They describe optical defects that go beyond simple spherical defocus and astigmatism.

There are no commercially available glasses that attempt to correct high order optical aberrations. The fundamental problem is that the glasses sit some distance away from the eye and the eye gazes at different portions of the lens at different times. Any complicated shape that correct wrinkles in the vision would only work if the eye was gazing exactly at the sweet spot.

Fortunately the amount of visual defect in almost every ones eyes is dominated by simple spherical defocus and astigmatism and glasses correct those well. Something less than about five percent of people have significant high order aberrations, some with really bad high order aberrations due to conditions like keratoconus.

Now I'm confused again. If it isn't "wrinkles" than what is an astigmatism condition in the eye?

Consider a plano-convex lens with a circular outline and a front surface which forms part of a sphere, and which is thickest in the centre of the circle.

This is a conventional lens, and will focus incident parallel light to a small spot at the focus.

Suppose the lens is made of soft transparent resin.  Squeeze the lens across a diameter, so that the outline is now elliptical, rather than circular.  The surface is curved more strongly in the direction you squeezed.  A thin sheet of parallel rays aligned with this diameter will be focused nearer to the lens than parallel rays in the same direction but in a thin sheet which is at right angles to this diameter.

There is no longer a single point where all parallel rays are brought to the same focus.  This is astigmatism, and arises when the curvature of the lens surface is different for different sections through the lens.  It can by corrected by aligning a cylindrical lens of the correct power with the direction of maximum or minimum curvature.

If there are multiple ridges or wrinkles in the surface of the lens, rather than a smooth ellipsoidal deformation, this cannot be corrected by a simple cylindrical lens.  Such defects are described as higher order aberrations.  As OpticsEngineer says, these are not usually corrected in spectacle lenses.

HTH

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Alan Robinson

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