The radius of an Airy disk is commonly defined to be the distance from its center to the first minimum and is equal to the lateral resolution of an objective lens. The numerical aperture of an objective lens is a measure of the angular size of the focusing cone of light as seen in Fig. The lateral resolution of a microscope also depends on the field of view. As the confocal pinhole size is decreased the field of view becomes vanishingly small the lateral resolution improves by a factor of . 1 or 2 will consist of in-focus parts and black or empty areas since the detector pinhole only accepts light from objects near the focal plane.
Axial resolution defines what near means.
Equation 3 gives an unambiguous expression for confocal lateral resolution. It is more difficult to obtain an equivalent expression for axial resolution, especially when dealing with photoluminescent or diffusely reflecting specimens. 3, and both formulas assume ideal or zero-size pinholes. At low NA Eqn. 4 agrees with Eqn. 5 except for a factor of two due to specular reflection. 4 and 5 should be considered only approximations which in some circumstances will fail to predict resolutions accurately.