Influence of spherical aberration, stimulus spatial frequency, and pupil apodisation on subjective refractions

Abstract

Purpose To test competing hypotheses (Stiles Crawford pupil apodising or superior imaging of high spatial frequencies by the central pupil) for the pupil size independence of subjective refractions in the presence of primary spherical aberration. Methods Subjective refractions were obtained with a variety of test stimuli (high contrast letters, urban cityscape, high and low spatial frequency gratings) while modulating pupil diameter, levels of primary spherical aberration and pupil apodisation. Subjective refractions were also obtained with low-pass and high-pass stimuli and using “darker” and “sharper” subjective criteria. Results Subjective refractions for stimuli containing high spatial frequencies focus a near paraxial region of the pupil and are affected only slightly by level of Seidel spherical aberration, degree of pupil apodisation and pupil diameter, and generally focused a radius of about 1 to 1.5 mm from the pupil centre. Low spatial frequency refractions focus a marginal region of the pupil, and are significantly affected by level of spherical aberration, amount of pupil apodisation, and pupil size. Clinical refractions that employ the “darker” or “sharper” subjective criteria bias the patient to use lower or higher spatial frequencies respectively. Conclusions In the presence of significant levels of spherical aberration, the pupil size independence of subjective refractions occurs with or without Stiles Crawford apodisation for refractions that optimise high spatial frequency content in the image. If low spatial frequencies are optimised by a subjective refraction, spherical refractive error varies with spherical aberration, pupil size, and level of apodisation. As light levels drop from photopic to scotopic, therefore, we expect a shift from pupil size independent to pupil size dependent subjective refractions. Emphasising a “sharper” criterion during subjective refractions will improve image quality for high spatial frequencies and generate pupil size independent refractions.

Changes in the Objective Amplitude of Accommodation with Pupil Size

“…The effect of the change in SA on the AA produced by the light conditions could be very important; 18 however, because subjective refractions tend to be paraxially biased, 25 a relatively small change can be expected, as occurred in our experiment (0.21 D).…”

Focus correction in an apodized system with spherical aberration

“…An interesting case would appear for a reversal SCE (γ < 0), with an amplitude function e −γρ 2 1 − e −γ , that assures a transmittance of 100% at the edge of the pupil, ρ 1 [10,30]. Figure 5(b) shows this case for A s > 0.…”

“…This interesting result shows the connection between SA4 and SA6 in terms of SR. This relation has been studied recently by means of numerical calculations [10]. After the appropriated defocus correction, SA6 may flatten the wavefront generated by the opposite sign SA4, thus increasing the concentration of light in the image plane [25,26].…”

“…The human eye exhibits a significant Gaussian amplitude filter (the Stiles-Crawford effect (SCE) apodization [7]) and significant levels of SA (more than typically encountered in fabricated optical systems [8]). In addition, identifying the defocus term that can maximize image quality in the human eye is a most critical clinical task, which has been shown to vary with both SA and apodization levels [9,10]. Extreme cases exist in eyes that have undergone corneal refractive surgery [11] and eyes implanted with refractive multifocal IOLs or with multifocal contact lenses for presbyopia correction.…”

“…The vision of these subjects is known to deteriorate with larger pupil sizes (typically found in night conditions) [12,13]. The change in refractive state that accompanies elevated levels of SA [10] is one possible reason for reduced visual performance. Therefore, the best refraction correction used during the day may not be the best to use at night [14].…”