Refractive index and axial distance measurements in 3-D microscopy

We use a simple geometrical model to show that when a converging wave is focused into a dielectric object, the axial movement of the object stage is not followed by the focus. This has important consequences for depth measurements in 3-dimensional microscopy. It was found for objects immersed in a watery solution that the apparent axial dimension can be up to three times larger than the actual size