Several structures of single particles have been resolved to 7-9 Å using electron cryomicroscopy. This field is now poised to make the next step, to near atomic (3-4 Å) resolution. Signal to noise ratio is the critical factor determining the amount of data required for a structural determination at higher resolution. When properly weighted, signal to noise ratio is additive as image data is averaged coherently, however, this relies on perfect alignment of the noisy images. Typical refinement algorithms require as high a signal to noise ratio as possible for accurate alignment. We report a method, which allows accurate, routine determination of signal to noise ratio and other image parameters, both for data quality assessment and microscope quality tracking.

Figure la shows a typical 400 kV electron image of ice-embedded herpesvirus capsids. The incoherent sum of the Fourier transforms of particle images (Fig. lb) shows that the data contains information out to a resolution of 7 Å. When rotationally averaged, this power spectrum can be expressed as: