By using subharmonic stroboscopic sampling method, very high resolution, comparable with that for discret mappings, has been reached in numerical study of periodically forced nonlinear oscillators. For the first time period-doubling bifurcation sequences up to 8192th subharmonics and corresponding sequences of chaotic bands are identified for a system, described by ordinary differential equations. Secondary and tertiary bifurcation sequences embedded in chaotic bands are shown to exist. Merits and limitations of this method as well as some precaution in using it are discussed briefly.