Families of "Optical Flow" methods are frequently used in the Computer Vision community to estimate the velocity from a pair of images, because of their ability to estimate a reliable and dense (i.e. one vector per pixel) motion field. Even if many extensions of usual PIV correlation-based methods are able to obtain more and more information associated to a "sub pixel" precision, let us remark that it is always difficult to estimate correctly the motion when some particular event appears such as a loss of pairing in the interrogation area, an image truncation due to its finite size, a velocity gradient in the interrogation window, ... Furthermore, only the "most probable" displacement is extracted. From this point of view, the idea of using a motion estimation technique based on optical-flow for fluid images is appealing. In this paper, a qualitative and quantitative study is done to compare motion fields obtained from an usual PIV method and with a particular optical-flow method, dedicated to fluid motion. A mixing layer and the near wake of a circular cylinder are analyzed and significant parameters are extracted for both methods. Statistical results are compared with hot-wire anemometry measurements.