Preferences are to be distinguished from operating conditions. Preferences are attached to the program, and remain the same for all lenses. Examples include items that affect the appearance of graphics windows, number formatting, and the like. Most preferences take effect as soon as they are set, but some require that the program be restarted. These include the maximum number of surfaces, spot diagram rays, and wavelengths that the program can use, as well as the selection of fonts. Once a preference has been set, it is saved in a configuration file (*.ini) that is read on startup, so the settings are preserved from one session to another.