The spectrum sampling out of focus of joint transform correlator has been discussed from three points of view, that is, theoretical analysis, computer simulation and experiment of the detection of defects in optical fibers. The relation curves between cross-correlation intensity and off-focus distance is obtained. The computer simulation results of joint transform correlation with different reference images and signal images in focus plane and off-focus plane respectively are demonstrated. The experimental results of detection for defects in fiber with different optical fibers in focus plane and off-focus plane are given.