Depth from defocus(DFD) measures defocus blur, which determines the point spread function (PSF) of an image formation system. Most DFD research has been investigated with a fixed focal length, where two or more defocus images are obtained with different image distances or lens apertures. To apply DFD techniques using a zoom lens, extensive calibration of the image formation system is needed. This work presents a simple calibration technique of defocus blur for a zoom lens system. We derive the relationship of defocus blur with respect to different focal lengths of the lens. The proposed technique is applied to a spatial domain DFD technique for error analysis.