Applications of confocal scanning microscopy and low coherence tomography in the biological and material sciences for optical-sectioning imaging require focusing light into the bulk specimen.However, the differences of refractive indices among biological tissues,the cover slip and immersion medium cause the strong spherical aberration and make the electromagnetic distribution in the focal region distorted.The spherical aberrations caused by focusing the light deeply into the specimen through multi-layer medium with different refractive index are analyzed. The results are used to calculate the limit penetration depth of microscopic imaging caused by focusing light into the bulk specimen through planar interfaces between materials of mismatched refractive index.The effect of spherical aberration on electromagnetic distribution in the focal region is also discussed.