107 research outputs found
Depth of focus of the human eye
No full textIn general optical systems, the range of distances over which the detector cannot detect any change in focus is called the depth-of-field. This may be specified by movement of the object or image planes, with the former being referred to as depth-of-field and the latter as depth-of-focus (DOF). Either term can be used in vision science, where we refer to changes in vergence which have the same value in both object and image space
Hypermetropia or hyperopia?
No full textA recent suggestion by a reviewer of a manuscript that the use of the word ‘hypermetropia’ was incorrect and that it should be replaced by ‘hyperopia’ caused us to look again at the literature of the subject to see if this criticism was justified. The background is an interesting one..
Do small-aperture presbyopic corrections influence the visual field?
No full textFree to read at publisher\ud
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Purpose\ud
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To explore the effect of small-aperture optics, designed to aid presbyopes by increasing ocular depth-of-focus, on measurements of the visual field.\ud
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Methods\ud
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Simple theoretical and ray-tracing models were used to predict the impact of different designs of small-aperture contact lenses or corneal inlays on the proportion of light passing through natural pupils of various diameters as a function of the direction in the visual field. The left eyes of five healthy volunteers were tested using three afocal, hand-painted opaque soft contact lenses (www.davidthomas.com). Two were opaque over a 10 mm diameter but had central clear circular apertures of 1.5 and 3.0 mm in diameter. The third had an annular opaque zone with inner and outer diameters of 1.5 and 4.0 mm, approximately simulating the geometry of the KAMRA inlay (www.acufocus.com). A fourth, clear lens was used for comparison purposes. Visual fields along the horizontal meridian were evaluated up to 50° eccentricity with static automated perimetry (Medmont M700, stimulus Goldmann-size III; www.medmont.com).\ud
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Results\ud
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According to ray-tracing, the two lenses with the circular apertures were expected to reduce the relative transmittance of the pupil to zero at specific field angles (around 60° for the conditions of the experimental measurements). In contrast, the annular stop had no effect on the absolute field but relative transmittance was reduced over the central area of the field, the exact effects depending upon the natural pupil diameter. Experimental results broadly agreed with these theoretical expectations. With the 1.5 and 3.0 mm pupils, only minor losses in sensitivity (around 2 dB) in comparison with the clear-lens case occurred across the central 10° radius of field. Beyond this angle, sensitivity losses increased, to reach about 7 dB at the edge of the measured field (50°). The field results with the annular stop showed at most only a slight loss in sensitivity (≤3 dB) across the measured field.\ud
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Conclusion\ud
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The present theoretical and experimental results support earlier clinical findings that KAMRA-type annular stops, unlike circular artificial pupils, have only minor effects on measurements of the visual field
Through-focus performance with multifocal contact lenses: Effect of binocularity, pupil diameter and inherent ocular aberrations
No full textPurpose: To evaluate the effects of the wearer's pupil size and spherical aberration on visual performance with centre-near, aspheric multifocal contact lenses (MFCLs). The advantage of binocular over monocular vision was also investigated. Methods: Twelve young volunteers, with an average age of 27 ± 5 years, participated in the study. LogMAR Visual Acuity (VA) was measured under cycloplegia for a range of defocus levels (from +3.0 to -3.0 D, in 0.5 D steps) with no correction and with three aspheric MFCLs (Air Optix Aqua Multifocal) with a centre-near design, providing correction for 'Low', 'Med' and 'High' near demands. Measurements were performed for all combinations of the following conditions: (1) artificial pupils of 6 and 3 mm diameter, (2) binocular and monocular (dominant eye) vision. Depth-of-focus (DOF) was calculated from the VA vs defocus curves. Ocular aberrations under cycloplegia were measured using iTrace. Results: VA at -3.0 D defocus (simulating near performance) was statistically higher for the 3 mm than for the 6 mm pupil (p = 0.006), and for binocular rather than for monocular vision (p <0.001). Similarly, DOF was better for the 3 mm pupil (p = 0.002) and for binocular viewing conditions (p <0.001). Both VA at -3.0 D defocus and DOF increased as the 'addition' of the MFCL correction increased. Finally, with the centre-near MFCLs a linear correlation was found between VA at -3.0 D defocus and the wearer's ocular spherical aberration (R2 = 0.20 p <0.001 for 6 mm data), with the eyes exhibiting the higher positive spherical aberration experiencing worse VAs. By contrast, no correlation was found between VA and spherical aberration at 0.00 D defocus (distance vision). Conclusions: Both near VA and depth-of-focus improve with these MFCLs, with the effects being more pronounced for small pupils and for binocular rather than monocular vision. Coupling of the wearer's ocular spherical aberration with the aberration profiles provided by MFCLs affects their functionality. © 2012 The College of Optometrists
Binocular summation improves performance to defocus-induced blur
No full textPURPOSE. To assess whether there are any advantages of binocular over monocular vision under blur conditions. METHODS. The effect of defocus, induced by positive lenses, was measured on the pattern reversal visual evoked potential (VEP) and on visual acuity (VA). Monocular (dominant eye) and binocular VEPs were recorded from 13 volunteers (average age, 28 ± 5 years; average spherical equivalent, -0.25 ± 0.73 D) for defocus up to 2.00 D using positive powered lenses. VEPs were elicited using reversing 10 arcmin checks (4 reversals/s). The stimulus subtended a circular field of 7° with 100% contrast and mean luminance 30 cd/m 2. VA was measured under the same conditions using ETDRS charts. All measurements were performed at 1 m viewing distance with best spectacle sphero-cylindrical correction and natural pupils. RESULTS. With binocular stimulation, amplitudes and implicit times of the P100 component of the VEPs were greater and shorter, respectively, in all cases than for monocular stimulation. Mean binocular enhancement ratio in the P100 amplitude was 2.1 in focus, increasing linearly with defocus to be 3.1 at +2.00 D defocus. Mean peak latency was 2.9 ms shorter in focus with binocular than for monocular stimulation, with the difference increasing with defocus to 8.8 ms at +2.00 D. As for the VEP amplitude, VA was always better with binocular than with monocular vision, with the difference being greater for higher retinal blur. CONCLUSIONS. Both subjective and electrophysiological results show that binocular vision ameliorates the effect of defocus. The increased binocular facilitation observed with retinal blur may be due to the activation of a larger population of neurons at close-to-threshold detection under binocular stimulation. © 2011 The Association for Research in Vision and Ophthalmology, Inc
'Les Perses de l'Occident' de Sotiris Skipis: Une invasion barbare en Europe dans les années 1920
No full textThe study examines the contribution of Sotiris Skipis (1881-1952) to theatrical art, an aspect which has been neglected by contemporary research. It focuses on an almost unknown three-act drama of Skipis, which the present research retrieved from the rare collections of Greek libraries: Les Perses de l' Occident (The Persians of the West). The author published the work in Paris in 1917, before the bloodshed of World War I was beginning to wane, to openly condemn pan-Germanism and racism, even before the latter term prevailed. The paper explores the ideological, aesthetic, and dramaturgical substratum of the text reflecting the spread of racial theories in Europe is explored. It also interprets the reasons that did not allow for the stage realization of the play on Parisian stages at the time of publication, despite the warm praise from Parisian scholarship, and that it took more than a century to pull the play out of oblivion
Magnocellular channel subserves the human contrast-sensitivity function
No full textThere is evidence that the human contrast-sensitivity function (CSF) is mediated by the spatiotemporal characteristics of magno and parvo neurons early in the visual pathway. In this study we use a measure of contrast gain derived from simple reaction times, to investigate the neural substrates of suprathreshold performance. The results reveal the activity of two mechanisms having distinctly different contrast-gain characteristics. Comparing these to neurophysiological data, we find that the magnocellular system dominates close-to-threshold detection and probably forms the basis of the achromatic CSF, whereas the parvocellular system dominates detection at higher contrasts, when the magnocellular system saturates. © 2005 a Pion publication
On-eye power characteristics of soft contact lenses
No full textBackground. The power of a soft contact lens on the eye is a function of its off-eye power, the manner in which the lens flexes on the eye, lens hydration changes, and the corneal topography. Methods. In the present study, we used a high illumination keratometer, which allowed us to obtain front and back surface keratometric readings when the lenses were in position on the eye. The on-eye power of the lens could then be calculated from these readings, with the assumption that the center thickness and refractive index of the lens corresponded to those in vitro. Results. The estimates of the on- eye powers agreed closely with the results indicated by over-refraction. Moreover, comparison of in vitro with in vivo power estimates indicated that the positive lenses lost power on-eye, whereas the negative ones maintained their power. Conclusions. The present study confirms the results of earlier workers, who suggested that soft lenses drape to fit the cornea. Our findings appeared to be in agreement with the predictions of most of the models developed in the past
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