It isn't about stopping motion
A few days ago I was having a conversation with another photographer and they brought up the idea of using high speed sync flash to stop motion. If I remember correctly, it was in regards to photographing liquid pouring out of a bottle into a glass. That got me thinking that the concept of hight speed sync (HSS) sounds like it might help, but that really isn't true.
Let's start with what HSS is. It allows you to sync your flash with shutter speeds above your normal sync speed so that you can use larger apertures for shallower depth of field or so that you can control the exposure on your background when working outdoors in full sunlight. A big minus is that HSS eats a lot of power and lowers the output of the flash and it eats through batteries (not an issue if you have an a/c powered flash).
What HSS does not do it help freeze motion. The two things that can help freeze motion are a fast shutter speed and/or a flash with a very short flash duration. High speed sync does let you use a faster shutter speed. But you no longer have a single fast flash burst. Instead, when your flash is in HSS mode it fires a number of times in rapid succession in sync with your camera’s shutter. The flashes are too close together for your eye to register them separately, so to your eye it looks like one flash, but it is actually strobing. At full power your speed light might take 1 to 3 or more seconds to recycle between flashes. In order to flash that many times in the short timespan of your shutter speed the flashes are relatively weak to allow the immediate recycling of the flash.
To show this, I’ve set up a couple of very basic experiments with a speed light and a Paul C. Buff Einstein studio flash (which offers an “action” mode with a very fast flash duration). Nothing very fancy, it is just an ordinary personal size desk fan, a camera, and a light. As a control, the first image is taken without any flash. It is lit by window light with the ISO cranked way up (256,000) to allow for a shutter speed of 1/4000 second. You can clearly see that 1/4000 of a second was not fast enough to freeze the blades on the fan.
Next is a series of images lit by a Canon 430 EXII speed light, off camera, connected by a remote cable. The first two images are taken at 1/5 of a second and 1/200 of a second (normal sync mode) and you can see that the flash did a fairly decent job of freezing the blades of the fan, but not quite good enough to make the label on one of the blades readable.
The next image is at 1/400 second, putting the flash into high speed sync mode. As you can see, it did a much worse job at freezing the blades of the fan. The next image moves the shutter speed up to 1/1250 of a second in HSS mode, and still a big blur. Even at 1/4000 of a second in HSS mode the label is a blur.
Compare the images at 1/4000 of a second with and without flash.
Now switch to the Einstein flash. The first two images have the Einstein in “color” mode which is intended to provide consistent color between flashes at the expense of a longer flash duration. You can see that at 1/2 second and at 1/160 of a second the label on the fan blade is not readily readable.
Changing the Einstein to “Action” mode to provide a shorter flash duration gives us the last two images, made at 1/2 and 1/160 second. Even these have a blurred label, but much sharper than the images made with high speed sync at 1/4000 of a second.
Now, I’m sure that most of us don’t have much occasion to photograph spinning fans. But we do sometimes get called on to photograph a liquid being poured. Does HSS help in this situation? I hope that you have already guessed the answer based on what you have read here so far. But let’s look at some images.
This time I will start with two using the Einstein. Each of these is at 1/2 second in “color” mode and in “action” mode. Both do a decent job of freezing the action of the pour and the bubbles.
Compare the above to the following images made with a speed light. The first one is at 1/60th second (normal sync) with good motion stopping. Next is at 1/500th second and we see that it is underexposed by a stop or two. Even with the flash being about 18-inches from the glass, it doesn’t have enough power to give me f/11. So, for the next image I bumped the ISO up from 100 to 400 to get a better exposure. Note that the action stopping is pretty much the same as at 1/60th of a second, at the cost of more noise due to the higher ISO (or you could give up depth of field and use a wider aperture—another tradeoff). Next I go to 1/2000 of a second, losing more power, but not gaining much, if anything, in the ability to stop action.
Here are two pour photos side by side. One is lit with a speed light in high speed sync mode at a shutter speed of 1/2000th of a second. The other is lit by an Einstein flash in normal sync mode at 1/60th of a second. Can you see much difference in them? Can you tell which one is at 1/2000th of a second?
So, there you have it. High Speed Sync is not helping you freeze motion. It is mainly used for outdoor photos where you want fill flash at wide open apertures for limited depth of field and/or to control the ambient exposure. For example, let's say your are making a portrait outdoors and you want to use an aperture of f/2.8 and on that particular day the shutter speed would need to be 1/2000. Without HSS or the use of neutral density filters you can't use your flash to fill in shadows. You would have to lower your shutter speed to 1/200 to sync, and that would need an aperture of f/9 (3-1/3 stops difference). Using HSS would allow you to photograph at 1/2000 @ f/2.8 and use your flash (in pretty close to the subject, as it won't be a very powerful flash). It will also allow you to go to 1/4000 @ f/2.8 to darken the ambient exposure while still giving the subject the correct flash exposure.
What about HSS in the studio. Is HSS valuable in a studio situation? That depends on the lights you are using. Personally, I can’t think of a reason to use HSS indoors with speed lights. In ETTL mode you can set the aperture you want and a low ISO to be able to take photos at wide open apertures. With more powerful studio flash units it will depend on how low you can set the power on the flashes. If you find yourself with your flash at minimum power and it is still giving you a smaller aperture than you want, then you can go into HSS mode and set your shutter speed above 1/250 and this will lower the power of the flash. Again, this is to control exposure, not to stop motion.
I hope this helps clarify things a bit. And, by the way, in the last image, A is 1/2000th of a second, B is 1/60th of a second.