Previous research has shown that there is a cost of dividing attention between the central and peripheral visual fields in a complex environment. However, it is not clear how stimulus factors, such as the contrast of the scene, affect the cost. The current study reports the results of two studies that address this question. In Experiment 1, temporal thresholds of the Useful Field of View (UFOV) tests were measured as a function of contrast and retinal eccentricity. The results showed that central-focused attention thresholds increased (i.e., performance decreased) as contrast decreased. Peripheral and divided attention task performance decreased as eccentricity increased. Surprisingly, peripheral and divided attention task performance were the best for medium rather than high contrast targets. The unexpected poorer performance under the high contrast condition might possibly be explained by the crowding effect. To test this possible explanation, in Experiment 2 the peripheral stimuli were simplified to minimize the potential crowding effect on peripheral target detection. The results showed that the unexpected effect of contrast on the cost of dividing attention could be accounted for by the crowding effect. When combined, the results from the two experiments suggest that the cost of dividing attention between central and peripheral targets is more pronounced for objects at greater eccentricity under lower contrast conditions, consistent with a tunnel effect. The implications of this finding are discussed in the paper.