Any eye discomfort caused by viewing something can be called eyestrain. Several causes are: reflective glare, improper corrective lenses, bright lights, improper viewing distance, and dry eyes. Computer usage can be a big contributor to eyestrain.
Eyestrain can become a major barrier to use of Adaptive Technology or Assistive Technology. Although the print may be large enough to be read for a period, the underlying cause of the visual impairment may cause the eye to fatigue quite quickly.