Still another type of a defect of the eye
is called astigmatism actually this can be a defect of any lens but in particular the
eye if it has astigmatism the curvature of the lens in one direction is different from
the curvature of the lens in another direction so in one plane the curvature has one value
and in another plane the curvature has another value and so you have a focal length for light
rays that are coming up for example in the horizontal plane they will come to focus at
a certain point light rays that are coming up in the vertical plane will come to focus
at a slightly different point and this is caused by a lens that is astigmatic or has
astigmatism an extreme example of a lens that has astigmatism would be a lens like this
now we would never want to use a lens like this for our eye lens because the curvature
this way is radically different than the curvature this way in fact it is flat across this way
and curved significantly this way there are various correcting procedures for lenses that
are astigmatic or have astigmatism among those include the application of combination lenses
as we have discussed with farsightedness and nearsightedness

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