1. An image capturing apparatus for performing digital processing on an image obtained by image capturing, the apparatus comprising: a storing unit configured to store, in advance of obtaining the image by image capturing, (a) a plurality of lattice point data sets for being set in a lattice point data table to perform conversion processing inside the apparatus and (b) a plurality of matrix coefficient sets as bases for a plurality of lattice point data sets for performing conversion processing with a matrix operation outside the apparatus, wherein each of the matrix coefficient sets is a basis for generating a lattice point data set in which a number of lattice points per one dimension is larger than a number of lattice points per one dimension in each of the stored plurality of lattice point data sets; a selection unit configured to select a lattice point data set from the plurality of lattice point data sets stored in said storing unit; a conversion unit configured to perform conversion processing on image data using the lattice point data set selected by said selection unit; and a file generation unit configured to generate an image file which includes (a) the image data, and (b) a matrix coefficient set stored in said storing unit, as attendant information in the image file, wherein the matrix coefficient set included in the image file corresponds to the lattice point data set which is selected by said selection unit and is configured to generate the lattice point data set by a matrix operation.