Understanding visual search and reading using the concepts of stimulus 'grain''

A simple calculation is presented which combines (i) a characteristic of visual stimuli (their 'visual grain') and (ii) two characteristics of the human observer (the central visual resolution and the scaling factor for eccentric vision), in order to make predictions about how manipulations of stimulus variables, such as size and stimulus quality, as well as observer variables, such as acuity and viewing distance, affect the visual span, that is, the amount of visual material that can be seen at a single fixation. Concrete applications to empirical data are shown where the calculation is used to explain how visual span, reading, and visual search depend on viewing distance, stimulus size, spacing, and viewer acuity.