Why can higher magnification produce empty magnification?

Empty magnification occurs when a microscope's magnification is increased without a corresponding increase in the numerical aperture (N.A.) of the objective lens.1

In microscopy, the ability to resolve fine detail is fundamentally limited by the diffraction of light. The N.A. of an objective determines its capacity to collect the diffracted light waves necessary to reconstruct an image of the object's structure. If an objective has an insufficient N.A. to capture the necessary diffracted waves, the fine details of the specimen cannot be resolved.21

When this limit is reached, further increasing the magnification—for instance, by using a higher-power eyepiece—simply enlarges the existing image without revealing any additional detail. Because the optical system lacks the resolution to distinguish the finer structures, the resulting image appears larger but remains blurry or indistinct, a condition known as empty magnification.1