What does cylinder mean and what is prism
Often the refraction of the eye (cornea and lens together) is not equally strong in all directions. This is called astigmatism. The glass of the glasses must then correct more in one direction than in the other. One speaks – especially with contact lenses and implant lenses – of a toric lens; in practice, however, there is often talk of a cylinder component or simply cylinder. The glasses prescription indicates how strong the cylinder should be and in which direction the cylinder axis should point.
When the eyes don’t work well together, a lot of effort has to be made to avoid double vision. This is called strabismus. If it is still possible, with difficulty, to make the eyes look in the same direction, the strabismus is latent; however, this can cause headaches and fatigue. The complaints can often be reduced by grinding a prism shape in the glasses. The bottom line is that the front and back surfaces of the lens are tilted slightly relative to each other. The beam of light is therefore bent slightly to the side. The squinting eye can then look relaxed while still seeing the correct image.