Recently, Franke et al.introduced a way to estimate the axial position of single molecules (temporal radial-aperture-based intensity estimation (TRABI)). To this end, they compared the detected photon count from a TRABI estimation to the estimated count from Gaussian point-spread function (PSF) fitting to the data. Empirically they found that the photometric ratio is ~0.7–0.8 at points close to focus and decreases as the distance from the focal plane increases. Here we explain this reported but unexplained discrepancy and, furthermore, show that the photometric ratio as an indicator for axial position is susceptible even to typical optical aberrations.

Light microscopy, allowing sub-diffraction-limited resolution, has been among the fastest developing techniques at the interface of biology, chemistry, and physics. Intriguingly no theoretical limit exists on how far the underlying measurement uncertainty can be lowered. In particular data fusion of large amounts of images can reduce the measurement error to match the resolution of structural methods like cryo-electron microscopy. Fluorescence, although reliant on a reporter molecule and therefore not the first choice to obtain ultraresolution structures, brings highly specific labeling of molecules in a large assembly to the table and inherently allows the detection of multiple colors, which enables the interrogation of multiple molecular species at the same time in the same sample. Here, the problems to be solved in the coming years, with the aim of higher resolution, are discussed, and what polarization depletion of fluorescence at cryogenic temperatures can contribute for fluorescence imaging of biological samples, like whole cells, is described.