High speed flash synch question

I was under the impression that High speed flash synch could be used to freeze very fast moving subjects using the flash unit & higher than normal shutter speed, but have been informed that the flash remains lit for a longer period than normal while the shutter slit remains open.
My question is, how is this possible if while using normal flash firing the shutter synch speed is 1/250th sec & if using HSS, the shutter speed could be 1/1000th sec or even higher?
Also, would using flash at reduced power work better than HSS to freeze movement?

Canon's Speedlites have a special mode called "High Speed Sync". When using this mode, the Speedlite emits a relative long duration (at least 1/200 second) string of very short (and rather weak) pulses of light. The net result is that this light acts very much like ordinary (continuously on) light during the time that the focal plane shutter is traveling across the film/sensor plane.

There are two blades to the shutter mechanism. With a slow shutter speed the bottom blade slides down, exposing the whole of the sensor, then the top blade follows, hiding the sensor. The camera flash happens while the whole of the sensor is exposed.

With a fast shutter speed the bottom blade starts moving down and, before it reaches the bottom, the top blade starts to follow it. A narrow slit travels down exposing parts of the sensor in turn. When the short flash goes off only a part of the sensor is exposed. A longer flash (or lots of short ones) is needed to ensure all of the sensor is exposed.

Thanks SkipD & Hollis_f.

hollis_f wrote in post #14615407
There are two blades to the shutter mechanism. With a slow shutter speed the bottom blade slides down, exposing the whole of the sensor, then the top blade follows, hiding the sensor. The camera flash happens while the whole of the sensor is exposed.

With a fast shutter speed the bottom blade starts moving down and, before it reaches the bottom, the top blade starts to follow it. A narrow slit travels down exposing parts of the sensor in turn. When the short flash goes off only a part of the sensor is exposed. A longer flash (or lots of short ones) is needed to ensure all of the sensor is exposed.

Very interesting. I didn't realize exactly how the shutter worked or the short bursts of flash.

Something else to think about here is that when not in hss mode, flash duration is in the neighborhood of 1/1000th of a second or even faster depending on the power used (lower power = faster). So if your flash is providing your subject the majority of its light, your 'effective shutter speed' is actually the flash duration. In this scenario you can safely 'drag the shutter' speed down to much lower than sync to bring up your background exposure and still have no problem freezing subject motion with the flash. Try playing around with this but remember that the vast majority of your subject exposure needs to come from your flash or else you will get ghosting. Dragging the shutter, as it’s called, is counterintuitive but very handy.

Also note that if the ambient light level is low, you can try an alternate approach:
Dial in exposure settings that have a relatively long shutter speed, yet leave the frame black (or very dark) if the flash doesn't fire. Low ISO, relatively small aperture, relatively long shutter speed (that is, anything slower than your sync speed of ~1/200).
If the scene is solely lit by the flash (no ambient) then the subject will still be frozen when the flash fires - even if the shutter speed is very long.

This is how many people accomplish specialty frozen-motion photography such as a bullet firing through an apple, a popping balloon, or a falling water droplet. Instead of trying to use a fast shutter speed, and time it just right to capture the moment, you just leave the shutter open and fire the flash(es) to grab the perfect moment.

[edit] Yeah, what Dustin said.

Also useful to know, is that more powerful flashes will often have a SLOWER effective shutter speed than lower-power flashes, because there's still a little bit of light being emitted as the flash is "cooling off" for that tiny fraction of a second. Some specialty flashes can have an effective shutter speed of 1/30000 or even faster when fired at low power levels. But even the slowest flashes are going to be like 1/1000 or faster, certainly enough to freeze most action.

HSS is really for being able to use fast apertures in bright light with flash while not having to resort to neutral density filters. Otherwise you will often find at something like f/1.4 and ISO 100 you cannot get the shot to not be overexposed without going over the synch speed.

HSS is not for freezing motion really, and in most cases where flash would be handy for a bit of fill while shooting action (sports for the most part) HSS kills the range to the point where it isn't effective.

If the flash is the prime source of light (not fill) then HSS is not desired at all, the flash duration itself will be much more effective at freezing action.

Thanks Dustin Mustangs, nathancarter & Jeffrey G. Excellent tips.