The human eye is an extremely delicate organ, and so performing ophthalmic surgery requires the ability to monitor progress on a micro-level. Ophthalmic operating microscopes are designed to provide high contrast and detailed imaging of all regions of the human eye. When choosing an ophthalmic operating microscope, it is important to pay attention to the type of optics employed. An apochromatic lens (or apo) will provide high light transmission, permitting high-quality imaging at lower light intensities. Certain models of ophthalmic surgery microscope provide multiple lighting options such as the ability to switch between halogen and xenon. An ophthalmic surgical microscope can either be fixed or adjustable, and some models provide a second “observer” set of binoculars, some of which can independently adjust the focusing mechanism.