Image filtering ([[band-pass filter]]ing) is often used to reduce the influence of high and/or low [[spatial frequency]] information in the images, which can affect the results of the alignment and classification procedures. This is particularly useful in [[negative stain]] images. The algorithms make use of fast Fourier transforms ([[Fast Fourier transform|FFT]]), often employing [[Normal distribution|Gaussian shaped]] soft-edged masks in [[Reciprocal lattice#Reciprocal space|reciprocal space]] to suppress certain frequency ranges. High-pass filters remove low spatial frequencies (such as ramp or gradient effects), leaving the higher frequencies intact. Low-pass filters remove high spatial frequency features and have a blurring effect on fine details.
