Human cornea is aspheric. Aspheric is a curve that is getting flatter by dimension as it goes from the center to the radius. It is like a rugby ball. On the whole, it makes an elliptical form. Compared with this, the normal lens on market has the same curve from the center to the radius like baseball or soccer ball shape. The human eye is aspheric, but if the contact lens is spheric, it is quite obvious that the eye will feel uncomfortable. That can cause discomfort and conjunctivitis through long time wearing.