Defect shape reconstruction ability of Synthetic Aperture Focusing Technique (SAFT) is investigated by numerical simulations. The numerical model employed is an infinite plate with an embedded cavity. Two types of cavities are considered. One is a circular cylinder and the other is a rectangular slit, both having the dimensions comparable to the incident wave length. Array sources/receivers of anti-plane waves are placed on a plate surface, and time domain scattered waveforms are calculated for every transmitter-receiver combination to simulate data collection by an SH-wave array transducer. Sets of array data simulated thus are fed into SAFT, and the cavities of given shapes are reconstructed. Comparing the resulting SAFT images one another, the effect of transducer position, source width and multiple wave paths on the shape reconstruction ability of SAFT is investigated. It is shown that aperture angle defined in the present study is an important concept to understand the effect of transducer position and of the multiple wave paths. It is also shown that the larger source width degrades SAFT images, but the degradation mechanism considering directivity of the incident field are explained.