David Johnson is an amateur photographer from Ottawa, Canada. He described the technique he used:





“When I saw a firework shooting in the air, I’d press the shutter button and hold in Bulb mode so that the exposure was precisely as long as it needed to be. When the explosion started, I would quickly refocus back to the fireworks (from 2ft in front of me). The exposure would start out of focus and slowly sharpen into focused points, which explains the strange shapes.” [Source]

For more photos you can also check out his portfolio on his personal website: www.daveyjphoto.com where you can see a great range of photography or his Flickr profile.