This textbook has been written to help trainee ophthalmologists in their professional examinations, covering the optical principles and applying them to clinical situations. The subject is approached from first principles, assuming no previous knowledge of optics. The text has been kept as logical and simple as possible and is liberally complemented with diagrams which are designed to be easily understood and memorised. Only the most fundamental and clinically useful mathematical formulae are quoted. Ophthalmology examinations have changed substantially since the last edition was written and a total rewrite of this essential guide has become necessary. The third edition has been made more comprehensive and detailed to keep up with these changes and maintain its place as the standard text on optics.