In wireless networks, a frame collision does not necessarily result in all the simultaneously transmitted frames being lost. Depending on the relative signal power and the arrival timing of the involved frames, one frame can survive the collision and be successfully received by the receiver. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that shows the terms and conditions (timing, power difference, bit rate) under which this capture effect takes place. A recent measurement work on the capture effect in 802.11 networks argues that the stronger frame can be successfully decoded only in two cases: (1) The stronger frame arrives earlier than the weaker frame, or (2) the stronger frame arrives later than the weaker frame but within the preamble time of the weaker frame. However, our measurement shows that the stronger frame can be decoded correctly regardless of the timing relation with the weaker frame. In addition, when the stronger frame arrives later than the weaker frame's arrival, the physical layer capture exhibits two very distinct patterns based on whether the receiver has been successfully synchronized to the previous weak frame. In explaining the distinct cases we observe that the successful capture of a frame involved in a collision is determined through two stages: the preamble detection and the frame body FCS check.