Shimming is a process in which minor adjustments are made to the magnetic field until the  field homogeneity (uniformity) is optimized. Improving the homogeneity will result in better  spectral resolution. It will be necessary to re-shim each time a probe or sample is changed. The system manager has stored appropriate shim values (in so called shim files) for each probe that will greatly reduce the shimming time required whenever a probe is changed.