As the bandwidth of SAR/ISAR system continuously increases as well as the synthetic aperture techniques develop, the spatial resolution of radar image is higher and higher. However, the high resolution of radar image does not mean the resolving ability for targets is high. We may have the same experience that the optical image is much easy to understand than radar image even if it has less resolution. A simple and frank explanation is due to the wavelengths of optical waves are much shorter than that of microwaves at which radars usually work, e.g., in the order of 10−5 –10−4 . Are there any the deep reasons for this? We will illustrate the inherent reasons in this paper from the aspects of imaging geometry and principle, electromagnetic scattering, signal characteristics, and so on.