In this paper a discussion on the difference between point-like targets and distributed scene in SAR images is presented starting from basic radar equations. The target detection requirements are formulated considering also the speckle characteristics of the distributed scattering of the background. An equation is presented for the estimation of the minimum radar cross section of a target to become detectable within a scene with speckled distributed background and detection criteria are discussed for the multi-look case. Experimental evaluations are based on airborne SAR data of a calibration test site.