> The resulting extraordinary photographs revealed intricate details of the first instant of an atomic explosion, including a few surprises such as irregular “mottling” caused primarily by variations in the density of the bomb’s casing. It also showed the detail of the “rope trick effect,” where the rapid vaporization of support cables caused curious lines to emanate from the bottom of an explosion.
The camera is interesting - using an electronic not mechanical shutter. This was around 1944. (Bell Lab's transistor was around 1947.)
More interesting still -- the mottling is from the remains of the casing and the shot cab (the little building the bomb was housed in at the top of the tower) splashing against the shock front at the fireball's boundary.