A lot of discussion in the keratoconus blogs I am a part of talk about how frustrating it is to not be able to see, even with glasses. People who do not have keratoconus don't understand why these patients are unable to use glasses to see well. Why they need special contact lenses. Keratoconus is a condition where the corneas become thinner and bulge forward, creating an ectasia. Here is a drawing to hopefully help explain.
To the left is a picture of a normal cornea. The light coming from the image hits the glasses and bends on to the cornea. The cornea is normal so the light is bent in a straight line. The light hits the retina resulting in a clear, sharp image.
To the left is a picture of the same situation, but in a person with keratoconus. Notice how the cornea is shaped like a cone, very steep, with irregularities.
The light that hits the glasses is still straight. However, once the light hits the irregular cornea, the light bends in all different directions, falling on the retina and creating an abnormal image.
As you can see, there is nothing that the glasses do to prevent an irregular image, other than sharpening it BEFORE it hits the cornea. This is why there are limitations to vision with keratoconus and glasses correction.