Last week, we showed what happens when you move the camera during the exposure: all points of light echo the path that the camera moved, and you get a collection of congruent streaks of light.  The effects shown in the last two weeks’ blog entries were achieved with a fixed focal length.

 

This time, we zoom (vary the focal length) during the exposure.  The resulting streaks are partially due to the explosion- and launch-trajectory-induced movement of the points of light, but the zooming can have a greater influence, causing the lights to streak outward (if zooming in during the exposure, or inward if zooming out during the exposure).  In some circumstances, zooming can produce a visual effect similar to a spaceship jumping to light speed or warp speed (as we’ve seen in Star Wars or Star Trek).

 

Typically, I will use a wide-angle zoom or a modestly-wide to modestly-telephoto zoom, and will zoom inward (from wide to telephoto) during the exposure.  I have had the best results when using a shutter speed just slow enough to capture the entire zoom; going slower than that will allow more of the firework’s explosion movement to influence the picture which, while not necessarily objectionable, does tend to dilute the effect of the zoom.

 

Although it takes a little more coordination to control two things at once, zooming can be combined with moving the camera to add yet another dimension of control to these images.  Also, with Photoshop, a composite image can be created by putting different fireworks images into layers, and then making the layers transparent, thus allowing them all to be seen simultaneously.

 

This hand-held image was taken with a 1/2s exposure and included principally a zooming-in effect (along with incidental camera movement caused by hand-holding the camera).  Had I used a tripod, the streaks would have been smoother because the camera movement would have been eliminated.  Had I used a faster shutter speed, the streaks would have been more linear (and shorter, if I did not zoom commensurately faster).