The resolution discussed here is affected by the diffraction of aperture light. The image formed by a point source is a diffraction pattern (Airy disk), with the central bright spot concentrating 83.78% of the total energy.

When the distance between two adjacent point sources equals the radius of the diffraction spot, the optical system can resolve them.

When the center-to-center distance between two adjacent diffraction spots in the image plane equals 0.85 (times the spot radius), the optical system can resolve them. The resolution is expressed as the shortest distance in the object plane between two distinguishable points.

In practice, the resolution criterion is proven to be somewhat conservative. It is generally referred to as the visual diffraction resolution of the optical system, or the ideal resolution.