We examined performance of healthy older and younger adults and individuals with Alzheimer’s disease (AD) and Parkinson’s disease (PD) on digit cancellation, a task putatively sensitive to cognitive impairment but possibly affected by visual impairment, particularly in contrast sensitivity. Critical contrast thresholds were established to create custom stimulus arrays that were proximally matched across individuals. Age- and PD-related differences in search were fully accounted for by the sensory deficit. Increased contrast benefited AD patients but could not override cognitive impairment. We conclude that visually-fair neuropsychological testing can effectively compensate for normal age- and PD-related visual changes that affect cognitive performance.