Contrast sensitivity is resilient to induced fast periodic defocus oscillations

Abstract

This study investigates the potential effects of periodic defocus oscillations on contrast sensitivity. Sinusoidal fluctuations at 5, 15, and 25 Hz, with defocus peak-to-valley values ranging from 0.15 to 3 D, were induced by means of a focus-tunable lens after calibrating its dynamic behavior. Monocular contrast sensitivity was measured on five young emmetropic subjects. The experimental data shows that contrast sensitivity loss due to defocus fluctuations is low for a wide range of frequencies and amplitudes. Only for the more severe case studied (25 Hz, ± 1.5 D) contrast threshold showed a clear increase in most subjects. Qualitative comparison of the empirical data with a simulation of modulation loss due to time integration of defocused retinal point spread functions, suggests a short integration time by the eye for defocus blur, around or even below a hundredth of a second.

Fig. 1.

Schematics (left) and actual implementation (right) of the optical setup based on an open-view HS sensor, with the addition of a tunable lens in front of the right eye for inducing defocus oscillations.

Fig. 2.

Contrast threshold as a function of defocus PtoV for 12-cpd 1-deg Gabor gratings seen through sinusoidal defocus oscillations induced by a tunable lens. Each graph corresponds to a subject (S1 to S5). Each color represents a temporal frequency of fluctuation (see legend).

Fig. 3.

Mean 12-cpd contrast threshold for 5, 15, and 25 Hz defocus oscillations as a function of PtoV, obtained by averaging across subjects the data points in Fig. 2. Error bars represent the standard deviation across subjects. Symbols for different frequencies were slightly shifted in horizontal direction to aid visibility of the error bars.

Fig. 4.

MTF at 12 cpd for static and dynamic (ramp) defocus. For the static case (orange line), the x-axis represents the amount of defocus. For the dynamic case (blue line), the x-axis represents the Max defocus in the sweep (the MTF comes from the average PSF in the range [-Max, +Max] D). Red lines illustrate the protocol for obtaining data points for Fig. 5.

Fig. 5.

Equivalence in blurring effects between static and dynamic (ramp) defocus: According to Fig. 4, a linear defocus scan with a Max value in the x-axis (range [-Max, +Max] D) produces the same drop in the 12-cpd MTF as the equivalent amount of static defocus in the y-axis.

Fig. 6.

Panels (a) to (e): Modulation loss (diffraction-limited MTF divided by integrated MTF) at 12 cpd, caused by defocus oscillations at 5 Hz (red), 15 Hz (green), and 25 Hz (blue), as a function of PtoV for different integration times. Panel (f): Modulation loss caused by whole range integration. Pupil diameter = 6 mm in all cases.