In the present study, we measured the extent of spatial summation in the detection of image contrast within the central 40° visual field. Contrast detection thresholds (in 28 observers) were measured for a spot of light of 10 different sizes [area: 0.03-1.92(°)(2)] at different retinal meridians (0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°) and eccentricities (0°, 5°, 10°, 15°, and 20°). Contrast detection thresholds were significantly affected by the size of the stimulus with sensitivity improving with stimulus size consistent with Ricco's law. Summation curves were similar across different spatial meridians, but the extent of spatial summation increased with retinal eccentricity consistent with previous reports. The size of the stimulus was also shown to affect contrast detection thresholds in the periphery. In particular, contrast detection thresholds decreased more rapidly with increasing eccentricity for a smaller target than a larger one. This difference in performance is accounted for by the accompanying change in Ac with eccentricity. In Experiment 2, we show that spatial uncertainty affected contrast detection, particularly at eccentric locations greater than 5°, such that cueing the location of the stimulus improved contrast thresholds. Spatial uncertainty improved overall performance but did not affect the estimates of the critical areas of summation. The results of the present study indicate that, due to spatial summation, detection performance is highly dependent on the size of the stimulus, its eccentric location, and spatial uncertainty. Future perimetric methodologies must consider these factors to improve detection sensitivity.