Ten years ago, German photographer Holger Keifel was looking through a few boxing books when he noticed that, in each photo, the boxer was in a similar position: hands up, tough face, ready to fight. He felt there was something missing — who the boxers were outside the ring.

Keifel resolved to change that. Now, he has become known in boxing circles for having what may be the largest single collection of boxing portraits in the world, consisting of 400 portraits of fighters, promoters, trainers, cutmen, and announcers. Every portrait tries to reveal the inner life of his subject, from Mike Tyson to Joe Frazier to Floyd Mayweather Jr.

Keifel shared some photos from his boxing project with us here, and you can see more in his book, "Box: The Face of Boxing." The National Portrait Gallery also recently acquired two of Keifel’s portraits.