The present paper introduces a method for measuring the atmospheric coherent time which is based on Fast Defocus method (FADE). The configuration of the system is modified in which, instead of an annular aperture, a mask with 8 or 4 regular apertures is used. In FADE there is an optics behind the telescope to make an annular image of the star at focus but in the current method, the image of the star is recorded at defocus to make 8 or 4 spots depends on the number of apertures. The average radial distance of the spots is used to estimate the defocus aberration coefficient same as the ring radius in FADE. Then the measured defocus aberration coefficient at a fast frame rate is used to calculate the Fried parameter, wind speed, and subsequently the coherence time, by fitting a model function to the temporal structure-function of the defocus. This method is called “modified FADE”. In order to evaluate the proposed method, FADE and modified FADE methods are compared using simulation. It is shown that the discrepancy in estimated Fried parameter and coherent time are within a few percent for both. The configuration of the modified FADE with 4 apertures is very similar to 4- DIMM defocus velocity method therefore to evaluate the proposed method experimentally, the coherent time of the Iranian National Observatory is measured by modified FADE and 4-DIMM defocus velocity methods. The discrepancy of the measured Fried parameter, wind velocity, and coherent time were 1, 3, and 5 percent.