Richard Calver | Anglia Ruskin University (original) (raw)

Papers by Richard Calver

Research paper thumbnail of Effect of aging on the monochromatic aberrations of the human eye

Journal of The Optical Society of America, 1999

We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and... more We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and compared the results with those from a group of younger subjects. We also measured each subject's monochromatic ocular wave-front aberrations using a crossed-cylinder aberroscope and calculated their modulation transfer functions (MTF's) and root-mean-squared (RMS) wave-front aberrations for fixed pupil diameters of 4 mm and 6 mm and for a natural pupil diameter. The CS at a natural pupil diameter and the MTF computed for a fixed pupil diameter were found to be significantly poorer for the older group than for the younger group. However, the older group showed very similar MTF's and significantly smaller RMS wave-front aberrations compared with the younger group at their natural pupil diameters, owing to the effects of age-related miosis. These results suggest that although monochromatic ocular wave-front aberrations for a given pupil size increase with age, the reduction in CS with age is not due to this increase.

Research paper thumbnail of Reassessing the theoretical accuracy of the crossed-cylinder aberroscope technique

Journal of The Optical Society of America A-optics Image Science and Vision, 1999

Research paper thumbnail of Effect of aging on the monochromatic aberrations of the human eye

Journal of The Optical Society of America A-optics Image Science and Vision, 1999

We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and... more We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and compared the results with those from a group of younger subjects. We also measured each subject's monochromatic ocular wave-front aberrations using a crossed-cylinder aberroscope and calculated their modulation transfer functions (MTF's) and root-mean-squared (RMS) wave-front aberrations for fixed pupil diameters of 4 mm and 6 mm and for a natural pupil diameter. The CS at a natural pupil diameter and the MTF computed for a fixed pupil diameter were found to be significantly poorer for the older group than for the younger group. However, the older group showed very similar MTF's and significantly smaller RMS wave-front aberrations compared with the younger group at their natural pupil diameters, owing to the effects of age-related miosis. These results suggest that although monochromatic ocular wave-front aberrations for a given pupil size increase with age, the reduction in CS with age is not due to this increase.

Research paper thumbnail of The spherical aberration of the crystalline lens of the human eye

Research paper thumbnail of Unequal Reduction in Visual Acuity with Positive and Negative Defocusing Lenses in Myopes

Optometry and Vision Science, 2004

Myopes have a reduced accommodative response to negative lenses compared with nonmyopes. Mathemat... more Myopes have a reduced accommodative response to negative lenses compared with nonmyopes. Mathematical models predict that the reduced accommodative response is due to a decrease in sensitivity to blur in myopes. We examined the effect of blur induced by positive and negative defocusing lenses on visual acuity in 12 myopes and 12 nonmyopes during cycloplegia for up to +/-3.00 D of defocus in 0.25 D steps. Although nonmyopes showed a symmetrical reduction in visual acuity with positive and negative lenses, the myopic group showed less acuity loss with negative lenses compared with positive lenses. The magnitude of visual acuity loss was lower with negative lenses in myopes compared with nonmyopes. No significant difference in visual acuity with positive lenses was found between myopes and nonmyopes. Residual accommodation (after cycloplegia) was about 0.20 D in both myopes and nonmyopes and was too small to explain the relatively good visual acuity through minus lenses in the myopic group. The reduced accommodative response known to occur in myopes may be due to the relatively small effect that negative lens blur has on their visual acuity.

Research paper thumbnail of Effect of defocus on contrast sensitivity in myopes and non-myopes

Ophthalmic and Physiological Optics, 2002

... effect of erroneous custom corneal correction on the modulation transfer function L. Diaz-San... more ... effect of erroneous custom corneal correction on the modulation transfer function L. Diaz-Santana,a N. Daviesb and D. Lara-Saucedoc aApplied ... Induced irregular astigmatism in hypermetropic PRK and LASIK: a bilateral cohort study A. Sciscio,a CC Hull,b CG Stephenson, ...

Research paper thumbnail of Increasing negative spherical aberration with soft contact lenses improves high and low contrast visual acuity in young adults

Ophthalmic and Physiological Optics, 2009

Purpose: To evaluate the effects of a customised manipulation of spherical aberration (SA) on th... more Purpose: To evaluate the effects of a customised manipulation of spherical aberration (SA) on the high and low contrast visual acuities and contrast sensitivity of young adult myopes.Methods: A placebo-controlled double masked trial of customised spherical aberration controlling soft contact lenses was undertaken in myopes aged 15–23. Participants wore customised soft contact lenses with either (i) zero spherical aberration or (ii) negative spherical aberration that resulted in a net SA (eye plus lens) of −0.1 micron. High and low contrast log MAR visual acuities and Pelli Robson contrast sensitivity were assessed in 196 eyes of 98 subjects after a period of 12 months wearing the lenses.Results: Both high and low contrast acuities were significantly better in the group wearing the contact lenses with negative spherical aberration (high contrast log MAR, p = 0.043; low contrast log MAR, p = 0.043) which was not due to differences in residual astigmatism or pupil size between the two groups. Pelli Robson contrast sensitivity was not significantly different in the two groups.Conclusions: Manipulation of spherical aberration, taking account of the participants’ baseline level of aberration, can cause statistically significant improvements in high and low contrast distance visual acuity although these improvements are too small in magnitude to be of clinical significance.

Research paper thumbnail of The effect of spherical aberration in myopic and non-myopic eyes: developing an optical model

Research paper thumbnail of The effect of altering spherical aberration on the static accommodative response

Ophthalmic and Physiological Optics, 2009

Purpose: To investigate the effect of altering the spherical aberration (SA) of the eye on the s... more Purpose: To investigate the effect of altering the spherical aberration (SA) of the eye on the static accommodative response.Methods: Participants were fitted with nominally afocal contact lenses with controlled amounts of SA of either −0.2, −0.1, 0.0, +0.1 or +0.2 μm for a 5-mm pupil. Measurements of SA and root mean square (RMS) total aberration for the eye plus lens for each participant were determined with a Complete Ophthalmic Analysis System aberrometer. Accommodation was stimulated either by placing targets at different dioptric distances from the eye, or by using a fixed distance target and placing negative-powered lenses in front of the eye. Accommodation responses were determined with a Shin-Nippon autorefractor.Results: For both stimuli situations, the slope of the accommodation stimulus-response function was lowest for the lenses with +0.2 μm SA, and increased as the amount of SA was reduced. There was a significant negative correlation between SA and slope. Lag of accommodation at 33 cm correlated well with added SA, but did not correlate with total RMS error. There was no significant difference between the responses at 30 min after lens wear started and the responses after 1 h.Conclusions: Adding negative SA to the eye generally improves the slope of the accommodation stimulus-response curve and decreases lag of accommodation, and positive added SA depresses the slope of the stimulus-response curve and increases lag. The effect seems to be specific to SA, as there was no relationship between lag and RMS error. Altering SA may be a viable way of changing accommodative functions in clinical situations.

Research paper thumbnail of Peripheral refraction for distance and near vision in emmetropes and myopes

Ophthalmic and Physiological Optics, 2007

We investigated the relationship between myopia and peripheral refraction for distance and near v... more We investigated the relationship between myopia and peripheral refraction for distance and near vision by measuring peripheral refractive errors in 10 myopic and 10 emmetropic participants at viewing distances of 2.5 and 0.4 m. Measurements were made at the fovea, and at eccentricities of 10°, 20° and 30° in the temporal and nasal hemispheres of the horizontal visual field. Our results showed that peripheral astigmatism increased with increasing eccentricity, but there was no significant difference between refractive error groups except at 30° eccentricity in the temporal retina. Considering the Mean Spherical Equivalent errors, emmetropes became relatively myopic at peripheral eccentricities, but there was little change in myopes. The effect of viewing distance on astigmatism or Mean Spherical Equivalent error was not significant. Our results do not support the view that myopia is associated with changes in peripheral refraction during distance or near vision.

Research paper thumbnail of Effect of positive and negative defocus on contrast sensitivity in myopes and non-myopes

Research paper thumbnail of The Health and Social Security Act 1984 and the price of spectacles among corporate practices in the United Kingdom (1980-2007): a review

Ophthalmic and Physiological Optics, 2010

The Health and Social Security Act 1984 deregulated certain aspects of optometry in the United Ki... more The Health and Social Security Act 1984 deregulated certain aspects of optometry in the United Kingdom, including advertising and the supply of spectacles, in the hope that greater competition would reduce spectacle prices. The effects of this legislation are tested by plotting the mean prices of private spectacles purchased from corporate optometric practices, from 1980 to 2007. Historical evidence is used to gauge the effect on prices of other factors such as National Health Service (NHS) payments, sight test fees and pressure exerted by consumer organisations. The high prices in 1980 reduced markedly throughout the rest of the 1980s, remained low for most of the 1990s and rose between 1999 and 2003, before falling. Changes in price were associated with changes to the system of NHS payments and variations in private sight test fees, but prices have generally been lower since the deregulation of dispensing than they were before. Although the price of private spectacles remains heavily influenced by other factors, the deregulation of opticians’ services has benefited the public by maintaining lower prices, as intended by proponents of the legislation.

Research paper thumbnail of Effect of aging on the monochromatic aberrations of the human eye

Journal of The Optical Society of America, 1999

We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and... more We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and compared the results with those from a group of younger subjects. We also measured each subject's monochromatic ocular wave-front aberrations using a crossed-cylinder aberroscope and calculated their modulation transfer functions (MTF's) and root-mean-squared (RMS) wave-front aberrations for fixed pupil diameters of 4 mm and 6 mm and for a natural pupil diameter. The CS at a natural pupil diameter and the MTF computed for a fixed pupil diameter were found to be significantly poorer for the older group than for the younger group. However, the older group showed very similar MTF's and significantly smaller RMS wave-front aberrations compared with the younger group at their natural pupil diameters, owing to the effects of age-related miosis. These results suggest that although monochromatic ocular wave-front aberrations for a given pupil size increase with age, the reduction in CS with age is not due to this increase.

Research paper thumbnail of Reassessing the theoretical accuracy of the crossed-cylinder aberroscope technique

Journal of The Optical Society of America A-optics Image Science and Vision, 1999

Research paper thumbnail of Effect of aging on the monochromatic aberrations of the human eye

Journal of The Optical Society of America A-optics Image Science and Vision, 1999

We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and... more We measured the contrast sensitivity (CS) of a group of older subjects through natural pupils and compared the results with those from a group of younger subjects. We also measured each subject's monochromatic ocular wave-front aberrations using a crossed-cylinder aberroscope and calculated their modulation transfer functions (MTF's) and root-mean-squared (RMS) wave-front aberrations for fixed pupil diameters of 4 mm and 6 mm and for a natural pupil diameter. The CS at a natural pupil diameter and the MTF computed for a fixed pupil diameter were found to be significantly poorer for the older group than for the younger group. However, the older group showed very similar MTF's and significantly smaller RMS wave-front aberrations compared with the younger group at their natural pupil diameters, owing to the effects of age-related miosis. These results suggest that although monochromatic ocular wave-front aberrations for a given pupil size increase with age, the reduction in CS with age is not due to this increase.

Research paper thumbnail of The spherical aberration of the crystalline lens of the human eye

Research paper thumbnail of Unequal Reduction in Visual Acuity with Positive and Negative Defocusing Lenses in Myopes

Optometry and Vision Science, 2004

Myopes have a reduced accommodative response to negative lenses compared with nonmyopes. Mathemat... more Myopes have a reduced accommodative response to negative lenses compared with nonmyopes. Mathematical models predict that the reduced accommodative response is due to a decrease in sensitivity to blur in myopes. We examined the effect of blur induced by positive and negative defocusing lenses on visual acuity in 12 myopes and 12 nonmyopes during cycloplegia for up to +/-3.00 D of defocus in 0.25 D steps. Although nonmyopes showed a symmetrical reduction in visual acuity with positive and negative lenses, the myopic group showed less acuity loss with negative lenses compared with positive lenses. The magnitude of visual acuity loss was lower with negative lenses in myopes compared with nonmyopes. No significant difference in visual acuity with positive lenses was found between myopes and nonmyopes. Residual accommodation (after cycloplegia) was about 0.20 D in both myopes and nonmyopes and was too small to explain the relatively good visual acuity through minus lenses in the myopic group. The reduced accommodative response known to occur in myopes may be due to the relatively small effect that negative lens blur has on their visual acuity.

Research paper thumbnail of Effect of defocus on contrast sensitivity in myopes and non-myopes

Ophthalmic and Physiological Optics, 2002

... effect of erroneous custom corneal correction on the modulation transfer function L. Diaz-San... more ... effect of erroneous custom corneal correction on the modulation transfer function L. Diaz-Santana,a N. Daviesb and D. Lara-Saucedoc aApplied ... Induced irregular astigmatism in hypermetropic PRK and LASIK: a bilateral cohort study A. Sciscio,a CC Hull,b CG Stephenson, ...

Research paper thumbnail of Increasing negative spherical aberration with soft contact lenses improves high and low contrast visual acuity in young adults

Ophthalmic and Physiological Optics, 2009

Purpose: To evaluate the effects of a customised manipulation of spherical aberration (SA) on th... more Purpose: To evaluate the effects of a customised manipulation of spherical aberration (SA) on the high and low contrast visual acuities and contrast sensitivity of young adult myopes.Methods: A placebo-controlled double masked trial of customised spherical aberration controlling soft contact lenses was undertaken in myopes aged 15–23. Participants wore customised soft contact lenses with either (i) zero spherical aberration or (ii) negative spherical aberration that resulted in a net SA (eye plus lens) of −0.1 micron. High and low contrast log MAR visual acuities and Pelli Robson contrast sensitivity were assessed in 196 eyes of 98 subjects after a period of 12 months wearing the lenses.Results: Both high and low contrast acuities were significantly better in the group wearing the contact lenses with negative spherical aberration (high contrast log MAR, p = 0.043; low contrast log MAR, p = 0.043) which was not due to differences in residual astigmatism or pupil size between the two groups. Pelli Robson contrast sensitivity was not significantly different in the two groups.Conclusions: Manipulation of spherical aberration, taking account of the participants’ baseline level of aberration, can cause statistically significant improvements in high and low contrast distance visual acuity although these improvements are too small in magnitude to be of clinical significance.

Research paper thumbnail of The effect of spherical aberration in myopic and non-myopic eyes: developing an optical model

Research paper thumbnail of The effect of altering spherical aberration on the static accommodative response

Ophthalmic and Physiological Optics, 2009

Purpose: To investigate the effect of altering the spherical aberration (SA) of the eye on the s... more Purpose: To investigate the effect of altering the spherical aberration (SA) of the eye on the static accommodative response.Methods: Participants were fitted with nominally afocal contact lenses with controlled amounts of SA of either −0.2, −0.1, 0.0, +0.1 or +0.2 μm for a 5-mm pupil. Measurements of SA and root mean square (RMS) total aberration for the eye plus lens for each participant were determined with a Complete Ophthalmic Analysis System aberrometer. Accommodation was stimulated either by placing targets at different dioptric distances from the eye, or by using a fixed distance target and placing negative-powered lenses in front of the eye. Accommodation responses were determined with a Shin-Nippon autorefractor.Results: For both stimuli situations, the slope of the accommodation stimulus-response function was lowest for the lenses with +0.2 μm SA, and increased as the amount of SA was reduced. There was a significant negative correlation between SA and slope. Lag of accommodation at 33 cm correlated well with added SA, but did not correlate with total RMS error. There was no significant difference between the responses at 30 min after lens wear started and the responses after 1 h.Conclusions: Adding negative SA to the eye generally improves the slope of the accommodation stimulus-response curve and decreases lag of accommodation, and positive added SA depresses the slope of the stimulus-response curve and increases lag. The effect seems to be specific to SA, as there was no relationship between lag and RMS error. Altering SA may be a viable way of changing accommodative functions in clinical situations.

Research paper thumbnail of Peripheral refraction for distance and near vision in emmetropes and myopes

Ophthalmic and Physiological Optics, 2007

We investigated the relationship between myopia and peripheral refraction for distance and near v... more We investigated the relationship between myopia and peripheral refraction for distance and near vision by measuring peripheral refractive errors in 10 myopic and 10 emmetropic participants at viewing distances of 2.5 and 0.4 m. Measurements were made at the fovea, and at eccentricities of 10°, 20° and 30° in the temporal and nasal hemispheres of the horizontal visual field. Our results showed that peripheral astigmatism increased with increasing eccentricity, but there was no significant difference between refractive error groups except at 30° eccentricity in the temporal retina. Considering the Mean Spherical Equivalent errors, emmetropes became relatively myopic at peripheral eccentricities, but there was little change in myopes. The effect of viewing distance on astigmatism or Mean Spherical Equivalent error was not significant. Our results do not support the view that myopia is associated with changes in peripheral refraction during distance or near vision.

Research paper thumbnail of Effect of positive and negative defocus on contrast sensitivity in myopes and non-myopes

Research paper thumbnail of The Health and Social Security Act 1984 and the price of spectacles among corporate practices in the United Kingdom (1980-2007): a review

Ophthalmic and Physiological Optics, 2010

The Health and Social Security Act 1984 deregulated certain aspects of optometry in the United Ki... more The Health and Social Security Act 1984 deregulated certain aspects of optometry in the United Kingdom, including advertising and the supply of spectacles, in the hope that greater competition would reduce spectacle prices. The effects of this legislation are tested by plotting the mean prices of private spectacles purchased from corporate optometric practices, from 1980 to 2007. Historical evidence is used to gauge the effect on prices of other factors such as National Health Service (NHS) payments, sight test fees and pressure exerted by consumer organisations. The high prices in 1980 reduced markedly throughout the rest of the 1980s, remained low for most of the 1990s and rose between 1999 and 2003, before falling. Changes in price were associated with changes to the system of NHS payments and variations in private sight test fees, but prices have generally been lower since the deregulation of dispensing than they were before. Although the price of private spectacles remains heavily influenced by other factors, the deregulation of opticians’ services has benefited the public by maintaining lower prices, as intended by proponents of the legislation.