An eye test involves measuring the degree of focusing error in your eyes. A prescription describes the type of lense needed to correct the focusing errors. Once you have had an eye test you will most likely be provided with the following;
This indicated the strength of the corrective lens for myopia (-ve) or hypermetropia (+ve). It is measured in dioptres (D).
This indicates the strength of the lens to correct for astigmatism. Once again, it is measured in dioptres.
This represents the orientation of the cylinder. It ranges from 0 to 180 degrees and is the angle at which the lense is set into the frame.
The eyesight simulator uses these values to create a point spread function (PSF). A convolution is then performed between the PSF and an image to create a representation of your natural eyesight.