The limitations imposed on reading by low vision aids.

Introduction: The changes that occur in the reading eye movements of normally sighted readers were measured as they used hand magnifiers to identify how these devices contribute to the slow reading of visually-impaired patients.

Methods: Subjects inexperienced in magnifier use read texts containing two sizes of print, using hand magnifiers of two different powers, held at two different eye-to-magnifier distances. The effect of magnification (up to 13.5x) and field-of-view (FOV) (2-45 characters) could be assessed independently.

Results: Reading speed decreased with increasing magnification because the size of the saccades did not increase in proportion to the magnification: for a given level of magnification, decreasing the FOV and decreasing the viewing distance both reduce the size of the saccades even further. The overall reading speed is only slowed significantly when the FOV restriction is extreme (two characters' width).

Conclusions: Two mechanisms seem to be used spontaneously by normally sighted readers to mitigate the limitation of reading speed created by the shortened saccades: head movement in the direction of reading and retinal image slip during fixation.