W. N. Charman

Fluctuations in accommodation: a review

When a young observer attempts to accommodate steadily on a fixed stimulus, the norminally steady‐state response shows small instabilities or fluctuations (sometimes termed microfluctuations or oscillations). These fluctuations typically have an amplitude of a few tenths of a dioptre and a frequency spectrum extending up to a few Hertz. The properties of these fluctuations are described for various viewing conditions: pupil diameter, target vergence, target form, target contrast, and target luminance all influence the frequency spectra of the oscillations, as may anomalies of vision such as amblyopia. The possible roles that the fluctuations might play in the function of the accommodative system are discussed. It is suggested that the higher frequency components around 2 Hz may arise from the mechanical and elastic characteristics of the lens, zonule and ciliary body. Components at lower frequencies (<0.5 Hz) may be of more significance in the function of the accommodative control system.

Dependence of accommodation response on the spatial frequency spectrum of the observed object.

Abstract Experiments are described in which the monocular, steady-state, accommodation response to sinusoidal grating targets was assessed as a function of the spatial frequency of the grating and its vergence at the eye, using a laser optometer. At all levels of stimulus, the response is found to be dependent upon the grating frequency. At very low spatial frequencies the response is often substantially in error and is closely related to the accommodation exercised by the observer when viewing an empty field. At higher frequencies the response becomes more accurate and its exact value is probably dependent upon the observing conditions and upon the ocular aberrations and. other properties of the observer. Further experiments with Snellen targets suggest that the accommodation to a target with a complex spatial frequency spectrum cannot be predicted from a knowledge of that spectrum and of the observers response to its component frequencies, under the same viewing conditions. The significance of the findings to theories of accommodation is indicated.

Off-axis image quality in the human eye

Abstract A “double-pass” photo-electric method is used to measure the vertical and horizontal external line-spread functions of the human eye. Measurements are taken in white light across the central 80° of the horizontal retinal meridian of both eyes of one subject. Optical transfer functions are derived from the external line-spreads and the retinal light distributions are computed. The data show a substantial region of roughly constant optical quality, centred on the optical rather than the visual axis of the eye. This region has a total extent of ·5° and is surrounded by a deteriorating periphery. The observed variation in optical quality contrasts with the published peripheral acuity data in that their quite different gradients suggest that mid-peripheral image quality is markedly more than adequate for the visual acuity, despite the substantial oblique astigmatism. An approximately linear relationship between the external and retinal line-spread function halfwidths is demonstrated and an estimate is given of the variation, with eccentricity, in relative illuminance of the retinal image of a point source.

The depth-of-focus of the human eye for Snellen letters.

&NA; Snellen acuity is determined as a function of refractive error for two subjects under cyploplegia, using randomised test charts, constant test chart luminance and a series of artificial pupils. These results allow determination of the depth‐of‐focus of the eye for Snellen targets, and it is shown that both optical and retinal factors influence the depth‐of‐focus achieved. Depth‐of‐focus is found to increase with decreasing pupil diameter and visual acuity. The clinical significance of these results, particularly with respect to the precision with which refractive error may be determined, is discussed.

Subjective depth-of-focus of the eye.

An experiment is described in which the subjective depth-of-focus (DOF) of the eye, defined as the range of focusing errors for which the image of the target appears to have the same clarity, contrast, and form as the optimal in-focus image, was measured as a function of the size of high contrast (99%) Snellen Es for 5 trained subjects under cycloplegia. Mean DOF increased by approximately 60% as the size of the letter detail increased from −0.2 to 0.87 log min arc (Snellen equivalent: 6/3.8 to 6/45), although there were considerable intersubject variations. DOF declined with increasing pupil diameter, the mean total DOFs being 0.86, 0.59, and 0.55 D for 2-, 4-, and 6-mm pupils, respectively. In a second experiment, use of low (21%) contrast letters with a 4-mm pupil and 4 subjects marginally increased the DOF (by 0.08 ± 0.05 D); refraction also shifted in a myopic direction by a mean of 0.15 ± 0.06 D compared with the high contrast letters. A third experiment with four less-experienced subjects demonstrated the importance of instruction and training in any measurement involving judgment of just-perceptible defocus blur. The clinical implications of the results for measurements of refraction and amplitude of accommodation are discussed.

MEASUREMENT OF THE AXIAL WAVEFRONT ABERRATION OF THE HUMAN EYE

Abstract— An objective variation of Howland and Howlands subjective crossed‐cylinder aberroscope method of estimating ocular aberration is described. An ophthalmoscopic arrangement allows the distorted retinal image of the aberroscope grid to be directly photographed for later analysis. Results for 10 subjects show that the ocular wavefront aberration near the visual axis is rarely symmetrical about the pupil centre and that, as a result, the phase transfer function is generally non‐zero. Calculated modulation transfer functions at various pupil diameters broadly agree with those of earlier authors but suggest that marked differences occur between individual eyes when the pupil diameter exceeds about 3 mm.

Accommodation and color

Previous studies of the effects of color on the accommodation response are reviewed. The monocular, steady-state response to targets under various colors of illumination is investigated. It is shown that trained observers change their level of accommodation, when viewing a target at a constant distance, to compensate for the varying ocular longitudinal chromatic aberration as the color of the target is changed. Untrained subjects, however, may initially show inconsistent responses. Results in white and greeen are closely comparable. Dynamic aspects of these effects are illustrated and it is shown that the ocular longitudinal chromatic aberration increases slightly with accommodation. The results are related to current ideas on the accommodative system.

Aberrations and myopia

It has been suggested that high levels of axial aberration or specific patterns of peripheral refraction could play a role in myopia development. Possible mechanisms involving high levels of retinal image blur caused by axial aberrations include form deprivation through poor retinal image quality in distance vision, enhanced accommodative lags favouring compensatory eye growth, and an absence of adequate directional cues to guide emmetropization. In addition, in initially emmetropic eyes, hyperopia in the retinal periphery may result in local compensatory eye growth, which induces axial myopia. Evidence in support of these ideas is reviewed and it is concluded that, for any fixed pupil diameter, evidence for higher levels of axial aberration in myopes in comparison with other refractive groups is weak, making involvement of axial aberrations in myopization through image degradation at the fovea unlikely. If, however, some potential myopes had unusually large pupil diameters, their effective aberration levels and associated retinal blur would be larger than those of the rest of the population. There is stronger evidence in favour of differences in patterns of peripheral refraction in both potential and existing myopes, with myopes tending to show relative hyperopia in the periphery. These differences appear to be related to a more prolate eyeball shape. Longitudinal studies are required to confirm whether the retinal defocus associated with the peripheral hyperopia can cause patterns of eyeball growth which lead to axial myopia.

The optical system of the goldfish eye

Abstract Experiments are described in which the parameters of the various optical components of the eyes of both living and dead fish were measured. The results are used to construct a schematic eye for the goldfish; this then forms the basis of a discussion of various aspects of the goldfish visual system.

The effect of pupil centration and diameter on ocular performance

The variation in sphero-cylindrical refractive error with position in the human eye pupil is derived from the wavefront error of two subjects. The modulation transfer function is also calculated for 1, 2 and 3 mm pupils decentred 1 and 2 mm nasally and temporally. Whilst decentration causes relatively little difference in the M.T.F. at the smaller pupil size, it can produce marked degradation for the larger pupils which can be of significance in the experimental determination of contrast sensitivity or other visual functions.