We consider the problems of focal length and focal depth of subwavelength imaging via a silver slab of metallic superlens . The performance limit of the metallic superlens was associated with the losses in the metallic film. The transmittance through a metal film is quite low and decreases exponentially with the thickness of the metal film. In the visible wavelength region, the permittivity of Ag can be approximated by the Drude model, so it can be described as the plural permittivity. The real part (E') of permittivity of the metal slab has been preferably index matched to the host material, and the imaginary part (E'') is considered to prevent ideal reconstruction of the image. Because superlens are usually made of metals with significant intrinsic loss (E''>0), the image is blurred and it is regarded as an ultimate limitation to a near field perfect lens. The real part (E')and the imaginary part (E'') of permittivity of the metal slab is the function of the incident wavelength, so we discuss the relationship of the focal length , focal depth and the incident wavelength. We also derive the expression for the resolution limit of metallic lens and demonstrate that the area of its subwavelength performance is usually limited to the near-field zone.