We employ self-consistent field theory to calculate binodals and spinodals of tin asymmetrie binary polymer blend in a thin film. The film surface and substrate of the film attract both the same component via short-range interactions. The ratios of chain lengths, segment lengths, and segment volumes are chosen to match with experimental parameters for a mixture of deuterated polybutadiene (dPB) and polyisoprene (PI). The form and strength of the surface potentials cannot be determined directly from the experiments. Therefore, we study the phase behavior as a function of surface interaction strength and film width. We find an interesting interplay between spinodal instability and prewetting behavior that leads to four possible spinodals: The two spinodals on the dPB-rich side mark the stability of thin and thick PI enrichment layers near the surfaces, the two on the PI-rich side are associated with the attraction of the two formed dPB/PI interfaces and bulk-like composition fluctuations in the PI-rich phase. The prewetting behavior also influences the binodals on the dPB-rich side, Upon increasing the film thickness, we observe that the binodals approach their bulk location while the spinodals remain different from the spinodals in the bulk because, even in very thick films, the unstable modes are associated with interface fluctuations rather than bulk composition fluctuations. We further study scattering qualitatively by analyzing the free energy required to excite certain density modes and discuss consequences for the interpretation of scattering experiments.