Thanks for all the feedback on yesterday's shutter speed post here and on social media. Some folks wondered about the action stopping capabilities of the speed light. I only have one speed light to test with, a Canon 430 EXii. So here are some more photos of the fan.
First, I will start out with this series of photos taken with no flash. Here only the shutter speed of the camera is affecting the amount of motion blur in the fan blades. Shutter speeds range from 1/8 second to 1/1000 second using window light.
The next series was taken with the flash on Manual power control and the camera shutter set to 1/15th second. The first image is without flash to show the ambient light at 1/15 second at f/8. The images then progress from 1/64 power to full power. The f/stop was adjusted for each frame to give the same exposure.
This next series is the same as the above, except that the shutter speed was increased to 1/125 second. I want you to notice that most of the images look exactly the same, despite the three step difference in shutter speed. Where you will see a difference is in the low powered shots where the aperture had to be opened up to compensate for the low power flash, causing more ambient light to be recorded at 1/15th than at 1/125th. But the edges of the blades are still about the same, despite the ghosting.
Here are close-ups of the fan blades at 1/4 power on the flash. On the left is 1/15 second at f/11 and on the right is 1/125 second at f/11. The motion blur is the same. The flash duration is the effective shutter speed in the studio. The flash doesn't care how long the shutter is open, and the flash isn't affected by how long the shutter is open (as long as the shutter speed is at or below the sync speed). If the room was completely dark the camera shutter could be set to 30 seconds or more and the exposure would still be the same as at 1/125. The only light hitting the fan is the brief flash from the speed light.
One more series of photo and then I hope we're done with the fan and can get back to more creative endeavors. This time the flash is in eTTL mode at a range of shutter speeds. The aperture is the same (f/5.6) in the 6 flash lit shots, the output of the speed light is controlled by the eTTL circuitry. The seventh image is the fan at 1/2000 second with no flash for comparison.
One of the things to notice in this series is that at 1/15 and 1/125 (the first two images) the flash is in normal mode and the speed of the flash freezes the blades. Above 1/200 second the flash is in high-speed sync mode where the flash fires a series of flashes (too quick in succession for the eye to see) as the shutter curtains move across the sensor. Because of the rapid movement of the fan blades, they are in different positions for each of the flashes and the fan blades show more motion at the faster shutter speeds (1/250, 1/500, 1/1000 second). At 1/2000 second the shutter speed alone stops the blades, as you can see by comparing the last two images, both taken at 1/2000 second. One is with the and one is without the flash.
As pointed out by Paul in the comments on yesterday's post, most subjects probably won't be moving as fast as the fan is, so you might not get a lot of subject motion blur using high-speed sync. I am exaggerating the effect here to make the point that high-speed sync is a tool used to help balance outdoor exposures between flash and sunlight while maintaining large apertures for shallow depth of field. It isn't meant to help stop subject motion, and, as can be seen here, may actually show more motion in extreme conditions.