The major difference between these two lenses is the optical performance. The DG model features a more conventional, achromatic optical design and uses one element of Special Low Dispersion glass (SLD). Its sharpness and contrast are rated very highly, however it is not quite as good as the APO model, which uses an apochromatic optical design and uses three SLD glass elements. This model has been top-rated by independent photo magazines. The APO DG model is also somewhat more expensive than the DG model. If you plan to make very large enlargements from your negatives, slides or digital files, the APO DG lens may be your better choice.