# Absolute frequency units `f_a`

``f_a=0``corresponds to the zero Fourier term (zero frequency), while``f_a=0.5``corresponds to Nyquist frequency``f_N``.is the maximum spatial frequency in an image scanned with a pixel size p [Å] and in pixels**Nyquist frequency**``f_N=1/2``, while in Å``f_N=1/(2p) [1/Å]``.

For the pixel size p [Å], spatial frequency ``f_s`` [1/Å] is related to absolute frequency ``f_a`` by:

``f_s=f_a/p````f_a=f_s**p``

For an image size n pixels sampled at p Å, k'th Fourier pixel (0<=k<=n/2) is related to frequency by:

``f_s=k/n/p````f_a=k/n``

Resolution r [Å] (defined as inverse of spatial frequency) is:

``r=1/f_s=p**n/k``

For examples of code in which Fourier pixels are handled check sparx/filter.cpp.