Customers must pay attention to the lens base curve when buying contact lenses. Lens base curse must be similar to the user's cornea so the cornea can get enough oxygen and function normally. Be careful of that too loose or too tight lenses may cause harm to the eyes. Too loose lenses will blur the user’s vision or feeling uncomforted and too tight lens or overtime wear will make the cornea hypoxia, causing corneal edema or vascular proliferation.