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F. Thorn, J.–C. He, S. Hill; Quality of Retinal Images During Reading . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5860.
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© ARVO (1962-2015); The Authors (2016-present)
We have shown that certain basic rules apply for adapting reading distance and posture to different types of text and reading environments although many aspects of reading behavior vary from person to person (Hill et al, ARVO, 2005, 2006). In this study, we demonstrate how reading behavior combines with an individual's optical aberrations, pupil diameter, and accommodative lag to effect the clarity and legibility of text.
Defocus, pupil size, and the ocular aberrations of both eyes of 12 normal young adults who participated in a reading study (Hill et al poster) were measured with a Shack–Hartman aberrometer while the subjects read text. Monochromatic 2–D fast Tracy transforms (FFTs) were performed on text from a children's book and a textbook at different magnification levels to simulate the text sizes and reading distances used during reading. The VolPro (Sarver, Inc) imaging system was used to analyze the spatial spectra of the text. The spatial spectra of the images were then multiplied by the MTF of each eye taking into account viewing distance and accommodative lag.
At a 25 cm reading distance, the various texts had their highest spatial power in the vertical direction at 0.7 to 1.2 cpd due to line spacing with ringing at higher frequencies. In the horizontal direction, the text shows a range of frequencies peaking between 3 cpd and 8 cpd. The power then decreases up to spatial frequencies of 8 to 13 cpd. This frequency range increases with viewing distance. We find that subjects maintain a reading distance and accommodative lag that provides retinal contrast of at least 20% for key spatial frequencies without serious phase reversals despite large intersubject differences in optical aberrations, pupil size, accommodative lag, and reading distance. This contrast is adequate for maintaining almost maximum reading speed but rarely approaches the highest possible contrast levels that can be reached with perfect accommodation.
Observers compensate for degraded retinal images by adjusting their reading distance and accommodative lag so that clarity is adequate for fast reading. This usually does not approach the maximum clarity possible for the eye with perfect accommodation.
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