<p>The dark focus is a resting state of accommodation, which occurs when there are not enough stimuli for the eye to focus on. This means that the eye becomes more myopic and some people notice a blur for example at night time, more known as night myopia. In this study the dark focus in Swedish myopes is measured and any difference between early and late onset myopes is investigated.</p><p><strong>Method: </strong>The dominant eye of 56 myopes was first measured with static retinoscopy, using a distant target, and then with near retinoscopy, with the retinoscope beam as the target. The full working distance of 2.00 D was subtracted in both methods and the values were compared. The difference, if any, was the dark focus.</p><p><strong>Results: </strong>The mean value of dark focus was 0.53 D ± 0.26 for the entire group. The mean value for early onset myopes was 0.56 D ± 0.29 and for late onset myopes the mean value was 0.47 D ± 0.21. This showed no significant difference (p-value = 0.18). No significant correlation between amount of refractive error and dark focus was found.</p><p><strong>Conclusion: </strong>The Swedish myopes in this study have a smaller mean value of dark focus than mean values found in other studies using the same technique and the previous findings that early and late onset myopes differ in mean values of dark focus is not applied to this study.</p>

The dark focus is a resting state of accommodation, which occurs when there are not enough stimuli for the eye to focus on. This means that the eye becomes more myopic and some people notice a blur for example at night time, more known as night myopia. In this study the dark focus in Swedish myopes is measured and any difference between early and late onset myopes is investigated. Method: The dominant eye of 56 myopes was first measured with static retinoscopy, using a distant target, and then with near retinoscopy, with the retinoscope beam as the target. The full working distance of 2.00 D was subtracted in both methods and the values were compared. The difference, if any, was the dark focus. Results: The mean value of dark focus was 0.53 D ± 0.26 for the entire group. The mean value for early onset myopes was 0.56 D ± 0.29 and for late onset myopes the mean value was 0.47 D ± 0.21. This showed no significant difference (p-value = 0.18). No significant correlation between amount of refractive error and dark focus was found. Conclusion: The Swedish myopes in this study have a smaller mean value of dark focus than mean values found in other studies using the same technique and the previous findings that early and late onset myopes differ in mean values of dark focus is not applied to this study.

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Although the static measure of accommodation is well documented, the dynamic aspect of the resting state (dark focus) of accommodation is still unknown. Previous studies suggest that refractive error is minimal at the intermediate resting point of accommodation — i.e., at the dark focus distances. Additionally, aging is closely linked to increased refractive error. In order to assess the effects of age on dark focus distance and its utility in enhancing the visual information transfer rate, two experiments were conducted under nighttime condition (scotopic vision) in a laboratory setting. A total of forty participants with normal vision or corrected to normal vision were recruited from four different age groups (younger: 26.9±5.0 years; middle-aged: 50.7±4.8 years; young-old: 64.6±2.8 years; and old-old: 79.8±6.1 years). Each age group included ten participants. In Experiment I, the accommodative status of dark focus at the fovea was assessed objectively using the modified autorefractor, a newly developed method to continuously monitor the accommodation process. The mean dark focus distances for younger, middle-aged, young-old, and old-old adults were 64.5±6.6, 73.4±20.6, 84.4±29, and 92.1±33.4 cm, respectively. There was a significant difference between the dark focus distances among different age groups. Post-hoc analysis indicated that there were statistically significant differences among young and old-old, young and young-old, and middle-aged and old-old age groups. In Experiment II, the information transfer rate was determined while viewing a target at three different distances: 52 cm, 73 cm (current recommended reading distances) and the individual's dark focus. A set of randomized alphabet characters were presented on a visual display with a luminance level of 20 cd/m2 and ambient illumination level of 4 lux. To assess the information transfer rate, participants were asked to read a set of characters aloud with their fastest rate for three seconds. Three measurements of information transfer rate at each viewing distance at random were made. Results obtained from each viewing distance were collected and averaged. The results showed that the mean visual information transfer rate for younger, middle-aged, young-old, and old-old adults were 14.27±1.43, 10.58±2.25, 9.35±2.13, and 7.73±2.36 bits/sec, respectively. There were statistically significant differences at α < 0.05 in means and standard deviations of visual information transfer rate in young and old-old, young and young-old, young and middle-aged, and middle-aged and old-old age groups. The mean visual information transfer rate at 52 cm, 73 cm and individual dark focus were 11.08±3.10, 10.14±2.97, and 10.22±3.42 bits/sec, respectively. There were statistically significant differences at α < 0.05 in means and standard deviations of visual information transfer rate at different viewing distances at 52 cm and 73 cm, and 52 cm and individual's dark focus. However, there were no statistically significant differences in the interaction between age and viewing distance (F = 1.6818, P = 0.1378) on the amount of visual information transfer rate. In summary, the visual information transfer rate was not greater when presenting visual stimulus at the individual's dark focus as compared with two fixed recommended viewing distances (52 cm and 73 cm). The greatest amount of visual information gained was at 52 cm. Actual and potential applications of this study including specifications for designs were also discussed. / Ph. D.