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Danielle Natale, Nicole Ross, Chris Bradley, Boger Yuval, David Teitelbaum, Frank S Werblin, Robert W Massof; Independent effects of angular and relative distance magnification on reading speed in normal and low vision subjects. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2216.
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© ARVO (1962-2015); The Authors (2016-present)
To describe the effects of angular and relative distance magnification on reading speed in normal and low vision subjects.
A head mounted display (HMD) was used to present images in virtual reality of 25 different paragraphs of texts. Each subject then read each of the 25 paragraphs with total magnification ranging from 1x to 25x. Magnification of the text was achieved with all combinations of angular magnification (ranging from 1X to 5X) and relative distance magnification (also ranging from 1X to 5X). The time to read each paragraph was recorded and words per minute reading speeds were calculated for each combination of angular and distance magnification.
Angular and relative distance magnifications have independent effects on reading speed. In each normal subject, above the critical print size reading speed declined linearly with the sum of relative distance and angular magnification - not the product. Reading speed declined at a faster rate with increases in angular magnification than with increases in relative distance magnification. In low vision subjects, reading speed increased from threshold with increased angular or relative distance magnification until reading speed reached the ceiling defined by normally sighted subjects and then with further magnification reading speed declined following the normal function.
The decline in reading speed as magnification increases cannot be explained by decreased preview area alone because the effects of angular and relative distance magnification on reading speed are additive, not multiplicative. This phenomenon is present in both normal and low vision subjects. One potential explanation for the independent effects on reading rates for the two types of magnification could be that relative distance magnification requires larger head movements with increasing magnification, whereas angular magnification does not, and angular magnification increases the magnification of image motion velocity with head movements, whereas relative distance magnification does not. The results we observed with a HMD, which required head movements to navigate text that extended beyond the field of view, might generalize to other types of movements required to translate text when using conventional low vision devices (e.g., moving the print to read with a microscope or a CCTV or moving the magnifier across the print).
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