K correction is a process for converting measurements of astronomical objects into their respective rest frames. The correction acts on that object's observed magnitude. Because astronomical observations often measure through a single filter or bandpass, observers only measure a fraction of the total spectrum, redshifted into the frame of the observer. For example, to compare measurements of stars at different redshifts viewed through a red filter, one must estimate K corrections to these measurements in order to make comparisons. If one could measure all wavelengths of light from an object, a K correction would not be required, nor would it be required if one could measure the light emitted in an emission line.