https://en.wikipedia.org/wiki/Reid_Blackburn

Photographs by Blackburn did not survive the eruption, though some footage he took weeks prior to the eruption was discovered later.

Landsburg is the one everyone talks about, though, since his photos survived.

https://en.wikipedia.org/wiki/Robert_Landsburg

In the weeks leading up to the eruption of Mount St. Helens, Landsburg visited the area many times in order to photographically document the changing volcano.[6] On the morning of May 18, 1980, he was within a few miles of the summit. When the mountain erupted, Landsburg retreated to his car while taking photos of the rapidly approaching ash cloud.[7] Before he was engulfed by the pyroclastic flow, he rewound the film back into its case, put his camera in his backpack, and then laid himself on top of the backpack to protect its contents. His body was found 17 days later, buried in the ash with his backpack underneath.[8][9] The film was developed and has provided geologists with valuable documentation of the historic eruption.[10]