What limits axial resolution

Hi all,

I was wondering what factors determine axial resolution in spherical lenses. I know that axial resolution in conventional light microscopy at least is about half as good lateral resolution. I've read about the huygens-fresnel principle of diffraction and how this can account for the size of the airy disk at the image plane-determined by the wavelength of light and the size of the aperture. However can you use this principle to explain why axial resolution is only half as good as lateral resolution? A formula i've seen for axial resolution
R = Lambda x refractive index of medium/numerical aperture^2

This is fine but it doesn't really explain to me why this is the case.

Thanks for the help!