In this talk I will describe studies that explored the spatial distribution of attention with a measurement that is independent of performance - the pupillary light response (PLR), thereby avoiding various obstacles and biases involved in more traditional measurements of spatial covert attention. Previous studies demonstrated that when covert attention is deployed to a bright area the pupil contracts relative to when attention is deployed to a dark area, even though display luminance levels are identical and central fixation is maintained. We used these attentional modulations of the PLR to assess the spread of attention. Specifically, we examined the minimal size of the attentional window and how it varies as a function of target eccentricity and the nature of other non-target stimuli. We found that the size of the attentional window doesn’t seem to vary as a function of eccentricity, but it is affected by stimuli size. Finally, we also found that the spatial spread of attention is influenced by perceptual load: the attentional window was larger when load levels were low than when load levels were high. These findings demonstrate the flexibility and constraints of spatial covert attention.