Abstract
Abstract: :
Purpose: In eyes having pupils large relative to focal length, depth of focus is short and chromatic defocus is a problem. Many aquatic and terrestrial vertebrates have multifocal lenses that compensate for chromatic defocus. Such lenses are usually studied by scanning a laser beam through a meridional plane of the lens and subsequent model calculations of the lens' performance. We have developed two methods that allow direct visualization of multiple focal lengths and how different wavelengths of light are focused by fish lenses. Methods:Isolated lensesuse were used, since in the typical fish eye, the crystalline lens is the only refractive element. Direct imaging: a lens was placed on a support in a tank filled with PBS, and microbeads (100 nm) to increase scattering of light. The lens was oriented along its optical axis by aid of structures in the lens capsule. A band of white light (xenon arc lamp) was shone through the axial meridional plane of the lens and focused inside the tank. The cone of light exiting the lens was observed with a microscope and images were taken with a high–resolution digital camera. Schlieren photography: a fish lens was suspended by its ligament in a small transparent tank filled with PBS. The lens was oriented along the optical axis by aid of the sutures. White light (xenon arc lamp) was shone on the lens via a semi–transparent mirror. The lens focused the light on a white diffuse reflector. The illuminated spot was imaged by the lens through the semi–transparent mirror on a pinhole in a telecentric position. Out–of–focus wavelengths were blocked at the pinhole. The lens was imaged through the pinhole with a 3–chip CCD camera. The results of both methods were compared with results from laser scans. Results: We studied the lenses of 10 crucian carps (Carrassius carrassius). Both methods show in fine detail the highly complex structure of the lens. Direct imaging: The method reveals that fish lenses indeed can focus various wavelengths at the same distance from the lens, despite longitudinal chromatic aberration and short depth of focus. Schlieren photography: zones of different focal lengths (for monochromatic light) are seen as colored rings. The colors indicate which wavelengths are focused by different zones of the lens. Those zones could be correlated to structures in the spherical aberration determined by laser scanning. Conclusions:We have for the first time directly demonstrated that fish lenses are multifocal and can focus various wavelengths in the same plane. The newly developed methods are sensitive enough to determine differences between individual lenses. All three methods give consistent results, and in combination allow for an unambigouous interpretation.
Keywords: visual acuity • imaging/image analysis: non–clinical • optical properties