Inside SPHIRE, we define the spatial frequencies are handled in absolute units. This means that the spatial frequencies are expressed in term of pixels units, or absolute frequency $f_a$, instead of units of inverse distance. In this way $f_a=0$ corresponds to the zero Fourier term (zero frequency), while $f_a=0.5$ corresponds to Nyquist frequency $f_N$. . 'Nyquist frequency' is the maximum spatial frequency in an image scanned with a pixel size p [Å] and in pixels f_n_1_2}, while in Å f_n_1_2p_1_a}.

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.