When a blur in images was changed, we found a new depth effect in a DFD (depth-fused 3-D) display that consisted of two overlapping 2-D screens. In ordinary DFD displays, two images displayed on front and rear screens are identical, but their luminances are different. Observers can perceive an apparent 3-D depth depending on the luminance ratio between the front and rear images. The purpose of our study is to examine the depth perception cues of a DFD display when two identical images are made slightly different by changing a blur. We found that the depth was controlled by only changing the luminance distribution around the edges of an image without any change in the luminance ratio. We proposed a perception model for depth changed by a blur in this paper. The perceived depth was qualitatively evaluated using a predetermined value. We suggest that the binocular disparity between the effective edges defines the perceived depth.