In tetragonally distorted Fe_0.5Co_0.5 alloy films grown epitaxially on Pd(001), Ir(001), and Rh(001) substrates the crystal field locates the electronic states near the Fermi level (E_F) with one being below E_F and the other above E_F with an energy separation smaller than in bulk nondistorted material. This results in a strong uniaxial anisotropy and an easy magnetization axis perpendicular to the film plane up to the thickness up to which the films remain tetragonally distorted. The strongest perpendicular anisotropy is achieved when the Fe_0.5Co_0.5 films are grown on Rh(001) (c/a=1.24); it systematically decreases for Ir(001) (c/a=1.18) and Pd(001) (c/a=1.13) substrates. The phenomenon can be understood as a result of an increasing uniaxial anisotropy with an increasing c/a ratio up to the maximum at c/a=1.24 for which the maximum uniaxial magnetic anisotropy has been theoretically predicted.