Astigmatism can be found in short-sighted and long-sighted people alike, and is quite a common condition, but is not usually understood by the average person.
Basically it means that the eye has difficulty in focussing on objects at certain angles i.e. some of the spokes on a cycle wheel.
In some cases vertical lines i.e. walls, may appear to the patient to be leaning over like the tower of Pisa.
The explanation for this is that the eye, instead of being shaped like a golf ball in the normal eye, is shaped more like a rugby ball. This consequently some of the rays of light passing through the lenses of the eye miss falling onto the focal point of the retina, and as a result cannot be seen so clearly as other rays of light which are still able to fall on the focal point in the normal manner.
Astigmatism can be corrected by including another power curve onto an existing lens. To explain this in simple terms we must consider the following examples and diagrams.
A normal plus lens could be explained an being a section out off from a s shown in FIG.5
An astigmatic or cylinder lens as they are more correctly known would be a lens cut from a glass tube, as in FIG.6
These two lenses could be combined into one lens by working the additional cylindrical lens into the surface of the normal plus spherical lens so that at certain angles there would be more power in the lens than at other angles, as FIG.7 illustrates.
By combining these two lenses, all of the rays of light passing through the lenses are now brought together to form an image on the focal point of the retina. This giving sharp vision through 180 degrees, which means that we would be able to see all of the spokes on our cycle wheel clearly,
The same principle applies in the case of Minus powered lenses in that an extra cylinder power would be worked into the surface of the normal Minus lens.
The best way to recognise if a lens has a cylinder power combination is to look through the lens from the rear at a vertical line, for example the upright of a door or window, and rotate the lens. If the line moves from the perpendicular or appears to lean when the lens is rotated then there is certainly a cylinder power in the lens (See FIG.8)
This method of checking works equally well whether the lenses are plus or minus power. The other point to note is that the greater the amount of deviation from the vertical, the greater the power of the cylinder in the lens
If you are a spectacle wearer yourself, try to establish which type of lens you have and whether there is any cylinder correction combined into the lens, If you don't wear spectacles try the test on a relative or friend's spectacles.