The nominal speed of KODAK PROFESSIONAL T-MAX 400 Film is EI 400. It was determined in a manner published in ISO standards. Because of its great latitude, you can underexpose this film by one stop (at EI 800) and still obtain high quality with normal development in most developers. There will be no change in the grain in the final print, but there will be a slight loss of shadow detail and a reduction in printing contrast of about one-half paper grade.
When you need very high speed, you can expose T-MAX 400 Film at EI 1600 and increase the development time. With the longer development time, there will be an increase in contrast and graininess with additional loss of shadow detail, but negatives will still produce good prints. You can even expose this film at EI 3200 with a longer development time. Underexposing by three stops and using three-stop push-processing produces a further increase in contrast and graininess, and additional loss of shadow detail, but the results will be acceptable for some applications.