Background

Astigmatism develops from an irregular shape of the cornea or an irregular shape of the lens, which makes it impossible for the lens to accommodate a fully focused image. Usually the lens will accommodate to a relatively small area of the visual world. In the case of the cornea, the surface of the cornea may be unsymmetrical, and in the case of the lens, the shape of the lens may not be sufficiently spherical. In astigmatism, the cornea bends the light more strongly in one direction, say in the vertical direction, than it does in the direction that is at a right angle to the first direction, say the horizontal direction. Thus, no matter how the lens changes shape, an object in a particular location and orientation will not be in focus. Astigmatism makes seeing some orientations blurry, whereas others are still relatively clear. To correct this problem, a complex, cone-like lens is used to offset the differences, though it is often difficult to correct an astigmatism with eyeglasses alone. Sometimes contact lenses are used to directly correct the problem. More recently, LASIK (laser-assisted in situ keratomileusis) surgery is used to reshape the cornea to bend the light in the correct fashion.

In this activity you can modify images to look at how astigmatism alters how scenes in the world appear. The relatively clear orientation and blurry orientation can be different for different people. You can rotate the astigmatism to see how different orientations can influence perception. You start with an image similar to ones used to assess astigmatism and a couple of photographs. You can even try your own images.

NOTE:

These simulations are performed in real time. They are computationally intensive and might take some seconds do perform. It will depend upon the browser you use and the power of your device; however, this illustration has been successfully run on tablets and phones.

Instructions

Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Astigmatism: Select whether the person has astigmatism or not.
Angle: Select the orientation of the astigmatism in 15 deg increments.
Severity of Astigmatism: How strong is the astigmatism: mild, moderate, or strong.
How Display: See only the image with the astigmatism or have this image next to the image of a person with a spherical cornea where the person does not have astigmatism.
Choose a Preselected Image: Choose another image to examine from a preselected list.
Use Your Own Image: You can upload your own image.

Reset

Pressing this button restores the settings to their default values but does not change the image.