The ultimate resolution of an image acquired by an optical system (a telescope or microscope) is governed by the laws of diffraction and can be expressed as a limit in an optical transfer function (OTF). Typically, the OTF characterizing a given optical system is dominated by the physical properties of the principal optical element, for example the microscope objective. However, the optical “system” can be construed more broadly since the advent of fast digital imaging processing.
Using new strategies the OTF can be “extended” to extract previously undetectable high spatial frequency information, and thereby “see” finer detail, by numerical processing of an appropriate series of diffraction-limited images. We will discuss a number of the techniques that are now being employed to achieve this “Super Resolution,” including: the sparse excitation of individual fluorescent molecules (Localization: PALM, STORM, etc.), point spread function engineering via stimulated emission depletion (STED), and spatial frequency down-mixing (Structured Illumination: SIM). These new computational microscopies plus rapid advances in genetically-engineered fluorescent proteins make for exciting times in observational cell biology. We will review the physics behind the various super-resolution schemes and will also consider some recent biology results, compellingly illustrated.