The new method for enhancing the focus latitude in optical lithography, named FLEX (focus latitude enhancement exposure) has been investigated. In FLEX the imaging characteristics are greatly affected by several key parameters such as distance between adjacent focal planes and illuminator coherence factor. The effect of these parameters on optical image characteristics have been examined using computer simulations. The results of the simulations have been verified by applying FLEX to imaging of sub‐half‐micron patterns using a high numerical aperturei‐line stepper. This has confirmed the validity of FLEX in delineating feature sizes in the order of 0.3∼0.5 μm. In particular, the depth of focus for hole patterns is found to be increased more than ten times, if necessary, with no degradation in resolution. However, the high‐contrast resist process is necessary in applying FLEX to patterns likeL/S. FLEX will be a great help to future optical lithography with high NA and/or short wavelength optical systems.