An optical microscope can never resolve detail smaller than the diffraction limit.

Even a perfect optical lens cannot focus light into an infinitely small point.

Because light behaves as a wave, a tiny object appears through a microscope as a blurred spot rather than a perfect dot. If two objects are close enough that their blurred patterns overlap strongly, an ordinary microscope can no longer separate them.

Ernst Abbe described this diffraction limit in the nineteenth century. For conventional microscopy, the result is real and fundamental.

Over time, however, the rule was often restated more broadly: no optical microscope could ever extract detail below that scale.

Super-resolution microscopy changed the assumptions.

Some methods separate fluorescent molecules in time. PALM and STORM switch on only a sparse set at once, allowing individual positions to be estimated precisely and many measurements to be combined into a finer image.

STED takes a different approach: an additional beam suppresses fluorescence around a small central region, shrinking the area that can glow.

These methods do not abolish diffraction. The light from each emitting molecule still diffracts. They obtain extra spatial information by controlling when and where fluorescence occurs and by combining many measurements.

Resolution is still limited by labeling, photon counts, noise, sample movement, and the particular method used. Abbe’s result remains correct for conventional imaging; what failed was treating those conditions as unavoidable for every optical microscope.