The cameras were specially designed and armored to resist the blast. Inch thick armored glass and armored plating helped ensure the cameras survived long enough to record the images. Also, film cameras are resistant to the emp pulse that accompanies nuclear explosions. No electronics inside mean no damage, so the camera keeps running until the film runs out.
Think of the camera being built into an airplane’s black box. The plane may crash and burn, but the recorder survives because of its design.
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