Numerical diffraction from a bacterial colony was investigated from the viewpoint of applying the sampling criterion for both spatial and frequency domains. Once the morphology information of a bacterial colony was given, the maximum diffraction angle was estimated to reveal the minimum and maximum length of both the imaging and aperture domains. Scalar diffraction modeling was applied to estimate the diffraction pattern, which provided that two phase functions were contributing to the phase modulation: chirp and Gaussian phase functions. Optimal sampling intervals for both phase functions were investigated, and the effect of violating these conditions was demonstrated. Finally, the Fresnel approximation was compared to the angular spectrum method for accuracy and applicability, which then revealed that the Fresnel approximation was valid for both large imaging distances and longer wavelengths.