Multifocal lenses compensate for chromatic defocus in vertebrate eyes

The focal length of the vertebrate eye is a function of wavelength, i.e. the eye suffers from longitudinal chromatic aberration. Chromatic defocus is a particularly severe problem in eyes with high light-gathering ability, since depth of field is small due to a pupillary opening that is large in relation to the focal length of the eye. Calculations show that in such eyes only a narrow spectral band of light can be in focus on the retina. For the major part of the visual spectrum, spatial resolution should be limited by the optics of the eye and far lower than the resolving power achievable by the retinal cone photoreceptor mosaic. To solve this problem, fishes with irises unresponsive to light have developed lenses with multiple focal lengths. Well-focused images are created at the wavelengths of maximum absorbance of all spectral cone types. Multifocal lenses also appear to be present in some terrestrial species. In eyes with mobile irises, multifocal lenses are correlated with pupil shapes that allow all zones of the lens, with different refractive powers, to participate in the imaging process, irrespective of the state of pupil constriction. Accepted: 6 November 1998     

Optical quality of the ocular lens of the sea lamprey (<Emphasis Type="Italic">Petromyzon marinus</Emphasis>) during the mature and transformer periods of life

While larval sea lampreys exist as eyeless filter feeders for several years, they transform into free-swimming juveniles (transformers) that attach parasitically to prey fish as they develop sexual maturity. This study examines lamprey lens development and optics and, since the lens is often the only refractive component of an aquatic eye, the data also provide an indication of visual ability during transformer and adult periods of life. Seven adult sea lampreys (0.40–0.55 m) and eight transformers (0.15–0.18 m) were sacrificed, the eyes removed and lenses dissected, measured, and placed in an automated laser scanning instrument. Back vertex focal length (spherical aberration) was measured for 14 beam positions across each lens by using a digital camera to record the position of the refracted beam. Transformer lenses exhibit positive spherical aberration, with average focal lengths varying from about 2.40 mm near the lens center and 1.06 mm at the lens periphery. On the other hand, the lenses from adults are largely corrected for spherical aberration, with average focal lengths varying from 2.19 mm to 2.44 mm. This result indicates that the younger lenses do not have a gradient refractive index necessary to mitigate the aberration and that further study of this model may reveal the relation between lens embryology and the development of such a gradient.     

Sturgeons,sharks, and rays have multifocal crystalline lenses and similar lens suspension apparatuses

Crystalline lenses with multiple focal lengths in monochromatic light (multifocal lenses) are present in many vertebrate groups. These lenses compensate for chromatic aberration and create well‐focused color images. Stabilization of the lens within the eye and the ability to adjust focus are further requirements for vision in high detail. We investigated the occurrence of multifocal lenses by photorefractometry and lens suspension structures by light and electron microscopy in sturgeons (Acipenseriformes, Chondrostei) as well as sharks and rays (Elasmobranchii, Chondrichthyes). Multifocal lenses were found in two more major vertebrate groups, the Chondrostei represented by Acipenseriformes and Chondrichthyes represented by Elasmobranchii. The lens suspension structures of sturgeons, sharks, and rays are more complex than described previously. The lens is suspended by many delicate suspensory fibers in association with a ventral papilla in all groups studied. The arrangements of the suspensory fibers are most similar between sturgeons and sharks. In rays, the lens is suspended by a smaller ventral papilla and the suspensory fibers are arranged more concentrically to the lens. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Comparison of Visual Performance of Multifocal Intraocular Lenses with Same Material Monofocal Intraocular Lenses

Purpose

To compare the visual performance of multifocal intraocular lenses (IOLs) and monofocal IOLs made of the same material.

Methods

The subjects included patients implanted with either Tecnis® monofocal IOLs (ZA9003 or ZCB00) or Tecnis® multifocal IOLs (ZMA00 or ZMB00) bilaterally. We conducted a retrospective study comparing the two types of IOLs. The multifocal group included 46 patients who were implanted with Tecnis® multifocal IOLs bilaterally. The monofocal group was an age- and sex-matched control group, and included 85 patients who were implanted with Tecnis® monofocal IOLs bilaterally. Lens opacity grading, the radius of corneal curvature, corneal astigmatism, axial length and the refractive status were measured preoperatively. Pupil size, ocular aberrometry, distance, intermediate and near visual acuity, contrast sensitivity with and without glare and the responses to a quality-of-vision questionnaire were evaluated pre- and postoperatively.

Results

The uncorrected near visual acuity was significantly better in the multifocal group, whereas both the corrected intermediate and near visual acuity were better in the monofocal group. Contrast sensitivity (with and without glare) was significantly better in the monofocal group. The rate of spectacle dependency was significantly lower in the multifocal group. There were no significant differences between the two groups regarding most items of the postoperative quality-of-vision questionnaire (VFQ-25), with the exception that the patients in the monofocal group reported fewer problems with nighttime driving.

Conclusions

The multifocal IOLs used in this study reduced spectacle dependency more so than monofocal IOLs and did not compromise the subjective visual function, with the exception of nighttime driving.     

Differences in lens optical plasticity in two gadoid fishes meeting in the Arctic

Arctic and boreal/temperate species are likely to be evolutionary adapted to different light regimes. Currently, the boreal/temperate Atlantic cod (Gadus morhua) is coexisting with the native polar cod (Boreogadus saida) in the Arctic waters around Svalbard, Norway. Here, we studied light/dark adaptative optical plasticity of their eye lenses by exposing fish to bright light during the polar night. Schlieren photography, high-definition laser scanning and ray tracing were used to determine the optical properties of excised crystalline lenses. Both species have multifocal lenses, an optical adaptation for improved color vision. In polar cod, the optical properties of the lens were independent of light exposure. In the more southern Atlantic cod, the optical properties of the lens changed within hours upon exposure to light, even after months of darkness. Such fast optical adjustment has previously only been shown in a tropical cichlid. During the polar night the Atlantic cod lens seems to be unregulated and dysfunctional since it had an unsuitable focal length and severe spherical aberration. We present a system, to our knowledge unique, for studying visual plasticity on different timescales in relation to evolutionary history and present the first study on the polar cod visual system.     

Spherical aberration of the fish lens: interspecies variation and age

Summary Spherical aberration of the eyes of a spectrum of freshwater fishes was determined by photographing the refractive effects of excised crystalline lenses on multiple parallel split laser beams. In general, spherical aberration is minimized by the developmentally related variation in lens refractive index. However, spherical aberration is marked and non-monotonic in a non-visual species such as the bullhead. Furthermore, the size and variability of the aberration appears to be related to visual need as indicated by diet and feeding habits. For example, the lenses of predatory sight feeders such as the pike (Esox lucius) or rock bass (Ambloplites rupestris) are optically superior to that of an omnivorous feeder as the carp (Cyprinus carpio).The effect of age was tested by examining rock bass lenses from fish two to seven years of age. Lens quality, as indicated by the amount of change in posterior focal length for beams of varying eccentricity from the optic axis, is optimum in lenses from five year old fish. The significance of this variation in lens quality is uncertain and requires further study with greater attention to specimens of advanced age.     

Active Focal Length Control of Terahertz Slitted Plane Lenses by Magnetoplasmons

Active plasmonic devices are mostly designed at visible frequencies. Here, we propose an active terahertz (THz) plasmonic lens tuned by an external magnetic field. Unlike other tunable devices where the tuning is achieved by changing the plasma frequency of materials, the proposed active lens is tuned by changing the cyclotron frequency through manipulating magnetoplasmons (MPs). We have theoretically investigated the dispersion relation of MPs of a semiconductor?Cinsulator?Csemiconductor structure in the Voigt configuration and systematically designed several lenses realized with a doped semiconductor slab perforated with sub-wavelength slits. It is shown through finite?Cdifference time?Cdomain simulations that THz wave propagating through the designed structure can be focused to a small size spot via the control of MPs. The tuning range of the focal length under the applied magnetic field (up to 1?T) is ??3??, about 50% of the original focal length. Various lenses, including one with two focal spots and a tunable lens for dipole source imaging, are realized for the proposed structure, demonstrating the flexibility of the design approach. The proposed tunable THz plasmonic lenses may find applications in THz science and technology such as THz imaging.     

A fibrous membrane suspends the multifocal lens in the eyes of lampreys and African lungfishes

The sharpness and thus information content of the retinal image in the eye depends on the optical quality of the lens and its accurate positioning in the eye. Multifocal lenses create well‐focused color images and are present in the eyes of all vertebrate groups studied to date (mammals, reptiles including birds, amphibians, and ray‐finned fishes) and occur even in lampreys, i.e., the most basal vertebrates with well‐developed eyes. Results from photoretinoscopy obtained in this study indicate that the Dipnoi (lungfishes), i.e., the closest piscine relatives to tetrapods, also possess multifocal lenses. Suspension of the lens is complex and sophisticated in teleosts (bony fishes) and tetrapods. We studied lens suspension using light and electron microscopy in one species of lamprey (Lampetra fluviatilis) and two species of African lungfish (Protopterus aethiopicus aethiopicus and Protopterus annectens annectens). A fibrous and highly transparent membrane suspends the lens in both of these phylogenetically widely separated vertebrate groups. The membrane attaches to the lens approximately along the lens equator, from where it extends to the ora retinalis. The material forming the membrane is similar in ultrastructure to microfibrils in the zonule fibers of tetrapods. The membrane, possibly in conjunction with the cornea, iris, and vitreous body, seems suitable for keeping the lens in the correct position for well‐focused imaging. Suspension of the lens by a multitude of zonule fibers in tetrapods may have evolved from a suspensory membrane similar to that in extant African lungfishes, a structure that seems to have appeared first in the lamprey‐like ancestors of allextant vertebrates. J. Morphol. 271:980–989, 2010. © 2010 Wiley‐Liss, Inc.     

A Focus on the Optical Properties of the Regenerated Newt Lens

Lens regeneration studies in the adult newt suggest that molecular aspects of lens regeneration are complete within 5 weeks of lentectomy. However, very little is known about the optical properties of the regenerated lens. In an aquatic environment, the lens accounts for almost all of the refractive power of the eye, and thus, a fully functional lens is critical. We compared the optical properties of 9- and 26-week regenerated lenses in the red spotted newt, Notophthalmus viridescens, with the original lenses removed from the same eyes. At 9 weeks, the regenerated lenses are smaller than the original lenses and are histologically immature, with a lower density of lens proteins. The 9 week lenses have greater light transmission, but significantly reduced focal length and refractive index than the original lenses. This suggests that following 9 weeks of regeneration, the lenses have not recovered the functionality of the original lens. By 26 weeks, the transmission of light in the more mature lens is reduced, but the optical parameters of the lens have recovered enough to allow functional vision.     

Investigation of Physical Limits of Plasmonic Focusing Lens in Different Regions

We present theoretical studies of three regions for plasmonic focusing, which are surface plasmon-dominating, Fresnel, and Fraunhoffer regions. The boundaries of the three regions are defined and the physical behaviors of plasmonic lenses in terms of focal length and focus size in these regions are investigated. A plasmonic lens that renders a subdiffraction-limit focus in the Fresnel region is presented and the lens performance with respect to the design parameters is studied by using finite-difference time-domain simulations. This work can serve as a basis for understanding plasmonic-focusing phenomenon and designing plasmonic lenses for various applications.     

Comparative x-ray diffraction study of the crystalline lens in a number of vertebrates including man

X-ray diffraction method has been applied for comparative investigation of native structure of eye lens proteins (crystallins). X-ray diffraction patterns of the whole lenses and/or their nuclear parts were obtained for man and vertebrate animals. Crystalline lenses of the fishes Acerina cernua and Pelmatochromis kribensis, frog Rana temporaria, bull and man contain crystallins with a very similar secondary and tertiary structure, whereas lenses of chicks and the tortoise Testudo horsfieldi contain mainly crystallins with other structure. The results obtained reveal evolutionary conservatism of crystallin structure in fishes, amphibians and mammals. It was also concluded that there is no correlation between crystallin structure of the lens, elasticity of the latter and accommodation mechanism.     

Patterns of pigmentation in the eye lens of the deep-sea hatchetfish,Argyropelecus affinis Garman

Summary The present study is a morphological, biochemical and spectrophotometric characterization of the eye lens pigmentation in 45 specimens (11–88 mm in standard length) of the deep-sea hatchetfish,Argyropelecus affinis (Stomiiformes: Sternoptychidae). For comparison, we also examined available lenses of other members of the family Sternoptychidae, including three other species of the genusArgyropelecus, and two species of the genusSternoptyx. Lens pigmentation was observed in all specimens ofArgyropelecus spp. larger than about 36 mm in standard length, but was absent in allArgyropelecus spp. individuals less than 36 mm. However, lens pigmentation was not observed inSternoptyx specimens of any size. Detailed studies ofA. affinis indicated that (1) at 36 mm the nascent lens fiber cells, which are continually laid down over preexisting, unpigmented cells, begin incorporating pigment, and (2) the pigment concentration increases steadily as pigmented cells are added during lens growth. Spectrophotometric and biochemical data suggested that the pigment is a carotenoprotein complex, the carotenoid-like chromophore being strongly associated with a specific soluble lens protein, alpha crystallin. While the lens coloration in these fishes is age-related, analyses of the retinal visual pigment revealed no concomitant age-related change in the peak wavelength of retinal sensitivity in these fishes. Our data on the spectral absorbance of the lens and visual pigment of these fishes suggest that the lens pigmentation acts as a short-wave filter to improve acuity of the visual system.     

Growth related changes to functional parameters in the bovine lens.

Dimensions, volumes and protein contents were measured for bovine lenses with wet weights ranging from 0.17-3.07 g (2 months gestation to 19 years post-natal). All increase in a non-linear fashion. The lens becomes flatter with age due to a more rapid increase in the equatorial plane, but the ratio of anterior to posterior sagittal distances remains constant (1.19). The radius of curvature increases from 4.9 to 15 for the anterior surface and from 4.4 to 13 for the posterior. Protein content increases more rapidly than volume resulting in an increased average protein concentration from around 18% in the early prenatal lens to nearly 50% in the 19 year old. Total protein content (TPC) was found to be related to wet weight (We) according to the equation, TPC = 0.3We1.33. It is suggested that TPC is a better parameter for describing growth than wet weight or age. The refractive index, in the equatorial plane, increases towards the centre, from 1.38 at the edge of the lens. The maximum index, in the centre, increases with lens size up to 1.474 in the largest lens studied. This corresponds to a protein concentration of 70%. In all lenses, refractive index and protein concentration gradients were superimposable when plotted from the outside towards the centre. The optical performance of the lenses was assessed by measuring the back focal length which increases gradually from 24 to 51.5 mm over the 0.17 to 3.07 g size range. This was attributed to the increased radii of curvature.     

Volume change of the ocular lens during accommodation

During accommodation, mammalian lenses change shape from a rounder configuration (near focusing) to a flatter one (distance focusing). Thus the lens must have the capacity to change its volume, capsular surface area, or both. Because lens topology is similar to a torus, we developed an approach that allows volume determination from the lens cross-sectional area (CSA). The CSA was obtained from photographs taken perpendicularly to the lenticular anterior-posterior (A-P) axis and computed with software. We calculated the volume of isolated bovine lenses in conditions simulating accommodation by forcing shape changes with a custom-built stretching device in which the ciliary body-zonulae-lens complex (CB-Z-L) was placed. Two measurements were taken (CSA and center of mass) to calculate volume. Mechanically stretching the CB-Z-L increased the equatorial length and decreased the A-P length, CSA, and lens volume. The control parameters were restored when the lenses were stretched and relaxed in an aqueous physiological solution, but not when submerged in oil, a condition with which fluid leaves the lens and does not reenter. This suggests that changes in lens CSA previously observed in humans could have resulted from fluid movement out of the lens. Thus accommodation may involve changes not only in capsular surface but also in volume. Furthermore, we calculated theoretical volume changes during accommodation in models of human lenses using published structural parameters. In conclusion, we suggest that impediments to fluid flow between the aquaporin-rich lens fibers and the lens surface could contribute to the aging-related loss of accommodative power. lens volume calculation; intralenticular fluid movement; presbyopia; mammalian lens     

High Efficient Far-Field Nanofocusing with Tunable Focus Under Radial Polarization Illumination

We design a new nanofocusing lens for far-field practical applications. The constructively interference of cylindrical surface plasmon launched by the subwavelength metallic structure can form a subdiffraction-limited focus, which is modulated by the dielectric grating from the near field to the far field. The principle of designing such a far-field nanofocusing lens is elucidated in details. The numerical simulations demonstrated that nanoscale focal spot (0.12λ ²) can be realized with 3.6λ in depth of focus and 4.5λ in focal length by reasonably designing parameters of the grating. The focusing efficiency can be 7.335, which is much higher than that of plasmonic microzone plate-like lenses. A blocking chip can enhance the focusing efficiency further as the reflected waves at the entrance would be recollected at the focus. By controlling the number of the grooves in the grating, the focal length can be tuned easily. This design method paved the road for utilizing the plasmonic lens in high-density optical storage, nanolithography, superresolution optical microscopic imaging, optical measurement, and sensing.     

Aquaporin 0 plays a pivotal role in refractive index gradient development in mammalian eye lens to prevent spherical aberration

Aquaporin 0 (AQP0) is a transmembrane channel that constitutes ∼45% of the total membrane protein of the fiber cells in mammalian lens. It is critical for lens transparency and homeostasis as mutations and knockout cause autosomal dominant lens cataract. AQP0 functions as a water channel and as a cell-to-cell adhesion (CTCA) molecule in the lens. Our recent in vitro studies showed that the CTCA function of AQP0 could be crucial to establish lens refractive index gradient (RING). However, there is a lack of in vivo data to corroborate the role of AQP0 as a fiber CTCA molecule which is critical for creating lens RING. The present investigation is undertaken to gather in vivo evidence for the involvement of AQP0 in developing lens RING. Lenses of wild type (WT) mouse, AQP0 knockout (heterozygous, AQP0^+/−) and AQP0 knockout lens transgenically expressing AQP1 (heterozygous AQP0^+/−/AQP1^+/−) mouse models were used for the study. Data on AQP0 protein profile of intact and N- and/or C-terminal cleaved AQP0 in the lens by MALDI-TOF mass spectrometry and SDS–PAGE revealed that outer cortex fiber cells have only intact AQP0 of ∼28 kDa, inner cortical and outer nuclear fiber cells have both intact and cleaved forms, and inner nuclear fiber cells have only cleaved forms (∼26–24 kDa). Knocking out of 50% of AQP0 protein caused light scattering, spherical aberration (SA) and cataract. Restoring the lost fiber cell membrane water permeability (P_f) by transgene AQP1 did not reinstate complete lens transparency and the mouse lenses showed light scattering and SA. Transmission and scanning electron micrographs of lenses of both mouse models showed increased extracellular space between fiber cells. Water content determination study showed increase in water in the lenses of these mouse models. In summary, lens transparency, CTCA and compact packing of fiber cells were affected due to the loss of 50% AQP0 leading to larger extracellular space, more water content and SA, possibly due to alteration in RING. To our knowledge, this is the first report identifying the role of AQP0 in RING development to ward off lens SA during focusing.     

A chiton uses aragonite lenses to form images

Hundreds of ocelli are embedded in the dorsal shell plates of certain chitons. These ocelli each contain a pigment layer, retina, and lens, but it is unknown whether they provide chitons with spatial vision. It is also unclear whether chiton lenses are made from proteins, like nearly all biological lenses, or from some other material. Electron probe X-ray microanalysis and X-ray diffraction revealed that the chiton Acanthopleura granulata has the first aragonite lenses ever discovered. We found that these lenses allow A. granulata's ocelli to function as small camera eyes with an angular resolution of about 9°-12°. Animals responded to the sudden appearance of black, overhead circles with an angular size of 9°, but not to equivalent, uniform decreases in the downwelling irradiance. Our behavioral estimates of angular resolution were consistent with estimates derived from focal length and receptor spacing within the A. granulata eye. Behavioral trials further indicated that A. granulata's eyes provide the same angular resolution in both air and water. We propose that one of the two refractive indices of the birefringent chiton lens places a focused image on the retina in air, whereas the other does so in water.     

A Design Method for a Micron-Focusing Plasmonic Lens Based on Phase Modulation

A design method of a micron-focusing plasmonic lens is proposed, which consists of a nanoaperture surrounded by concentric annular grooves with fixed width and depth. The phase modulation of the radiation lights decoupled from surface plasmon polariton waves by the annular grooves is realized by altering the radii of the grooves. Based on the principle of the constructive interference, a design formula of a micron-focusing plasmonic lens is deduced. The transmitted fields through the designed plasmonic lenses are numerically simulated with finite-difference time-domain method, and the results show that a circular focusing spot is generated where the focal length can be controlled in several micrometers, which agree with our theoretical analysis.     

Rapid,Accurate, and Non-Invasive Measurement of Zebrafish Axial Length and Other Eye Dimensions Using SD-OCT Allows Longitudinal Analysis of Myopia and Emmetropization

Refractive errors in vision can be caused by aberrant axial length of the eye, irregular corneal shape, or lens abnormalities. Causes of eye length overgrowth include multiple genetic loci, and visual parameters. We evaluate zebrafish as a potential animal model for studies of the genetic, cellular, and signaling basis of emmetropization and myopia. Axial length and other eye dimensions of zebrafish were measured using spectral domain-optical coherence tomography (SD-OCT). We used ocular lens and body metrics to normalize and compare eye size and relative refractive error (difference between observed retinal radial length and controls) in wild-type and lrp2 zebrafish. Zebrafish were dark-reared to assess effects of visual deprivation on eye size. Two relative measurements, ocular axial length to body length and axial length to lens diameter, were found to accurately normalize comparisons of eye sizes between different sized fish (R² = 0.9548, R² = 0.9921). Ray-traced focal lengths of wild-type zebrafish lenses were equal to their retinal radii, while lrp2 eyes had longer retinal radii than focal lengths. Both genetic mutation (lrp2) and environmental manipulation (dark-rearing) caused elongated eye axes. lrp2 mutants had relative refractive errors of −0.327 compared to wild-types, and dark-reared wild-type fish had relative refractive errors of −0.132 compared to light-reared siblings. Therefore, zebrafish eye anatomy (axial length, lens radius, retinal radius) can be rapidly and accurately measured by SD-OCT, facilitating longitudinal studies of regulated eye growth and emmetropization. Specifically, genes homologous to human myopia candidates may be modified, inactivated or overexpressed in zebrafish, and myopia-sensitizing conditions used to probe gene-environment interactions. Our studies provide foundation for such investigations into genetic contributions that control eye size and impact refractive errors.     

Sequence Characterization of γ-Crystallins from Lip Shark (Chiloscyllium colax): Existence of Two cDNAs Encoding γ-Crystallins of Mammalian and Teleostean Classes

γ-Crystallin is a common lens protein of most vertebrate eye lenses and the major protein component in lenses of fishes and in many mammalian species during embryonic and neonatal stages. To facilitate the structural characterization of γ-crystallin possessing extensive charge heterogeneity, a cDNA mixture was constructed from the poly(A)⁺ mRNA isolated from shark eye lenses, and amplification by polymerase chain reaction (PCR) was carried out to obtain cDNAs encoding multiple shark γ-crystallins. Sequencing analysis of multiple positive clones containing PCR-amplified inserts revealed the presence of a multiplicity of isoforms in the γ-crystallin class of this cartilaginous fish. It was of interest to find that two shark cDNA sequences coexist, one encoding γ-crystallin (γM₁) of high methionine content (15.5%) and the other encoding one (γM₂) of low methionine content (5.1%), each corresponding to the major teleostean and mammalian γ-crystallins, respectively. Comparison of protein sequences encoded by these two shark cDNAs with published sequences of γ-crystallins from mouse, bovine, human, frog, and carp lenses indicated that there is about 61–80% sequence homology between different species of the piscine class, whereas only 47–66% is found between mammals and shark. A phylogenetic tree constructed on the basis of sequence divergence among various γ-crystallin cDNAs revealed the close relatedness between shark γM₂-crystallin and mammalian γ-crystallins and that between shark γM₁ and teleostean γ-crystallins. The results pointed to the fact that ancestral precursors of γ-crystallins were present in the sharp lens long before the appearance of modern-day mammalian and teleostean γ-crystallins.