Question

The better resolution obtained with a high-numerical-aperature objective is accompanied by reduced contrast and depth of field. Why?

Answer 1

The bigger the cone of light that can be brought into the lens the higher it's numerical aperture is. Therefore the higher numerical aperture of a lens, the better the resolution of a specimen will be which can be obtained with that lens.

The second advantage of using the higher numerical aperture is that since more orders of the diffraction from the object are brought into the lens, more light generally is brought into higher numerical aperture lens producing brighter images.

At high numerical aperture of microscope, depth of field is determined primarily by wave optics, while at lower numerical aperture, the geometrical optical circle of confusion dominates the phenomenon. The total depth of field is given by the sum of wave and geometrical optical optical depths of field as: