It has been shown that effective uniaxial anisotropy fieldsHk″of epitaxial magnetic‐garnet bubble materials can be found from an optical measurement of the sample's easy‐axis susceptibility. The susceptibility is influenced by the cubic anisotropy which the materials possess in addition to their induced uniaxial anisotropy. This paper examines in detail how the optical measurements are affected by various ratios of cubic and uniaxial anisotropies. In particular, it is found thatHk″can be found accurately using this technique for a wide range of material parameters which includes bubble materials. A procedure which corrects for sample misalignment is presented along with a method by which the cubic and uniaxial anisotropy constants for a material can be calculated from measurements of itsHk″and magnetization.