it's been a long time since i've studied physics and physical chemistry so i could be wayyyy off but ...
how i understand it is two-fold. first, heat density. the surface area of the explosion is significantly smaller than the sun, and so the energy in the explosion is condensed to a smaller area and thus creates a higher temperature. as it expands it will, therefore, "cool" (relatively speaking) and by the time it reaches 7 miles away it wont melt the piss out of the camera (it obviously survived).
second is the spread of heat from the explosion to the camera. heat transfer is not instantaneous, and so the camera was just fine at this point. again, by the time it spreads to the camera it's dissipated so much energy that the heat will be relatively cooler.
finally i imagine the camera was pretty well protected to withstand the various forces of the explosion.