A comparative study of two different approaches to focal shifts

There are two different approaches to deal with focal shifts,i.e.,Li-Wolf's and Greene Hall's approaches.In this paper focal shifts in elegant(complex-argument)Laguerre-Gaussian(ELG)beams are taken as a typical example,a detailed comparative study of the two approaches is made and numerical calculations results are illustr ated.The conclusion made in this paper is useful for some practical applications.