Close-up photograph of a person's eye following a corneal graft (keratoplasty), an operation in which any diseased parts of the cornea (the membrane covering the eye) are replaced by clear corneal tissue from a donor. Keratoplasty may involve all layers of the cornea (penetrating keratoplasty) or only the superficial layers (lamellar keratoplasty). In the latter case the thickness of the donated cornea is reduced accordingly.