In stereoscopic displays, stereo depth is associated with parallax in parallax image, and parallax is related with the layout of cameras when the parallax images are captured. In the paper, these cameras are laid in parallel configuration. By setting proper distance between cameras and shifting the images reasonably, the objects with a certain depth have zero parallax and the objects with other depth have expected parallax. The parallaxes correspond to different stereo depths, so the quantitative relationship between stereo depth and distance of two cameras for capturing the parallax images is obtained. An experiment is carried out to verify the theory, the results show that proposed quatitative relationship can help to take good parallax images for stereoscopic displays by laying out cameras suitably.