Pupil size mainly responds to changes in illumination, likely to optimize vision for the given level of brightness, but it also fluctuates with changes in the alerting system. Recently, it has been argued that such pupil dilations also subserve a perceptual function: Due to the eye’s optics, detection performance in the periphery should be enhanced when pupils are larger, like during states of high effort. We investigated peripheral visual detection during somatosensory stimulation, that is physical effort. In three experiments, two with physical tasks, one with tactile stimulation, data confirmed that the larger the absolute pupil size, the better the peripheral visual detection performance. However, the stronger the relative pupil dilation caused by somatosensory stimulation, the lower the detection performance. The findings show that optical effects can be estimated by absolute pupil size, and effects of effort can be estimated by relative changes in pupil size. In sum, larger pupils benefit peripheral detection and higher states of effort go hand in hand with decreased detection performance. This suggests an intriguing relationship: pupil dilation under high states of effort may partially compensate for the herewith associated decrease in peripheral detection, thereby helping us to navigate in and interact with the environment.