Visualization of chromatic correction of fish lenses by multiple focal lengths

Many fish and some terrestrial animals have multifocal lenses that compensate for chromatic aberration by a mechanism unique to biological optical systems. We describe four optical methods for the study of such lenses. Significantly improved methods (focal-area imaging, schlieren photography, and laser scanning) are explained in full detail. One method (photorefractometry) was only slightly modified over earlier versions and is illustrated in brief. Portable equipment is available for photorefractometry, schlieren photography, and laser scanning. With our methods we could for the first time directly illustrate chromatic correction by multifocal lenses and detect a number of previously unknown optical features of fish lenses.