Panning Action Shots - Two Approaches

This weekend, we attended a fun birthday party where kids got to ride a zipline.  It would have been easy to freeze the action using flash, however I felt that a shot that captured the motion would more effectively convey the kids' excitement.  I used two approaches to do this. 

First Approach: Panning

The first approach is a more conventional approach, namely panning the shot.  Panning the shot means to use a slow shutter speed, then moving the camera in the direction of the subject's motion while taking the shot.  The effect is to keep the subject relatively sharp, while the background is blurred with motion.  Here's what it looks like:

The shot at the beginning is also an example of panning.  For both shots I used a shutter speed of 1/15.  Panning is quite challenging and sometimes, all you get is blur, like this:

To me, it's still more emotional than a sharp but completely static shot, but that's just my opinion.

Second Approach: Flash Blur + Panning

Another approach I used is to combine panning with flash blur.  Flash blur also uses a slow shutter speed to create blur, but keeps the subject sharp with the burst of flash.  More info on flash blur here.  Here's a sample shot:

For the flash blur effect to work as I intended, I underexposed the ambient, while keeping the flash on the subject normally exposed. If ambient is not underexposed, then the shot looks similar to regular panning.  I used shutter priority and selected a slow shutter speed of around 1/15.  I used exposure compensation to underexpose by about 1EV.  In Nikon, flash exposure takes exposure compensation into account, therefore I offset the underexposure with around +1 FEC.  Manual exposure would have worked as well: I would have selected 1/15 for shutter speed, then adjusted aperture until the light meter shows underexposure of 1 stop.  No offsetting FEC adjustments would have been needed.

With this flash blur approach, I got a much higher rate of acceptable shots.  The tradeoff compared to regular panning is that the background blur is less noticeable.

More samples of flash blur:

Sidenote: rear sync vs. front curtain sync
Usually, when using flash at slow shutter speed to show movement, using rear curtain produces a more natural result because the motion trail comes behind the subject.  I used rear curtain sync for the flash blur shots.  If you look at the background, you'll see that it's frozen, with the motion trail flowing to the right.  In this situation, because I was panning, I think front curtain sync would have looked ok too, though I didn't get to try it.