Why are there photographs of nuclear tests?

I believe most of the em-spectrum from a nuke is low energy. So when you take a photo you get a lot of visible light and heat, and only small amounts of high energy radiation on your film. Also the lens might be transparent to visible light, but non transparent to high-energy em-waves.

Edit: just found http://www.fas.org/nuke/intro/nuke/thermal.htm

It says that most of the inital photons are indeed in the X-ray spectrum, but these high energy photons are quickly absorbed by the atmosphere. So the spectrum reaching the photographer is determined by the atmosphere transparency.