Recent introduction of phase measurement interferometer (PMI), to measure wavefront aberrations brought about rapid reduction of residual aberrations in stepper and scanner projection lenses. Zernike sensitivity method (ZSM) is useful to capture and to understand the lens aberrations impact on the imaging performances, and to guide the improvements in patterning performance of the projection tools. In this paper we present ZSM for CD-Focus curve capable of precisely predicting CD at any focus position. We found that cross-term interactions of several Zernike combinations impact ZSM for CD-Focus. We present an example of V-H difference for which the cross-term interactions dominate. Aerial image simulation results presented here are illustrated by corresponding to experimental results.