Optics of Astigmatism

Optics of Astigmatism

This more technical page will help you understand the optics of astigmatism.

 

To begin, review the drawing below of a simple, non-astigmatic optical system.  

Figure 1: Non Astigmatic Optical System
Light
A
B
C
Figure 1:  A ray of light originates from a distant point source on the left.   The light passes through a lens (A) and focuses at a single point (B).   The rays diverge at distance (C).   Images are focused at B and appear fuzzy at C.  This represents the optics of a simple, non astigmatic optical system.
Figure 2:  Astigmatic Optical System
Light
A
B
C

Figure 2 demonstrates an astigmatic optical system.  An astigmatic optical system focuses light equally and simultaneously at two different distances.  This occurs because the refractive power of the optical system is greater in one dimension than the other (these two dimensions have been flattened into one in this diagram). Half of the image is focused at point (B) and half of the image is focused at point (C).  As a result, at both points B and C, light is focused and defocused.  Where is the best focus?  It depends on what you are doing.  It could be at point B, or C, or somewhere in between. Nowhere is perfect.  With astigmatism, images are always partially out of focus.  

Astigmatism is measured in the unit Diopter.  Less than 0.50 Diopter is a small amount of astigmatism that might not affect your daily life and may be hard for your surgeon to reliably correct.  More than 1.00 Diopter of astigmatism will cause noticeable blurring of your vision and can be reduced during cataract surgery.   For more information see the Astigmatism Simulator.

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