High speed sync flash question

[timmyquest]

I was reluctant to ask as i'm most likely going to just give it a go anyways...but here it is.

My rebel has a max sync speed of 1/200 as most know. I would like to see 1/250 or more speeds at the football field. The only way to do this obviously is to use highspeed sync.

It is my understanding that highspeed sync works by flashing pulses (upwards of 80hz) while the slit moves across the sensor.

It is also my understanding that "synced" flash photography works by flashing one powerfull pop while the shutter is fully open.

Now, what are the negative visual effects of using highspeed sync in low-light action photography.

Would the subjects come out blured?
Would the photographs be under-exposed?

[Scottes]

The photo will not be underexposed. E-TTL will keep you happy.

But it *is* possible that some things will come out blurred. I saw a site where a guy had a spinning disc showing the effects of high speed sync. The disc had a pattern on it, and sync'd flash froze it perfectly. But high-speed sync actually messes up the picture because of the flashes, and it definitely did not freeze the disc. It exposed the spinning disc several times along the spin so it looked quite odd.

When sync'd, a flash bursts incredibly quickly - 1/10000 sec, I think?? So that flash *will* freeze the subject. But it all depends on ambient light, too. If ambient light is high then you might get ghosting - though this can look way cool with 2nd curtain shutter. If ambient is low you'll get a black background. Not so good.

And what's the distance to the subject? The further they are the less flash that hits... If you're using telephoto (300mm+) then you might want to look at a Better Beamer which can boost flash output 2-3 stops.

[robertwgross]

Also note that when you run an external flash in high speed sync mode, you effectively cut down its guide number. In other words, its range is reduced.

It can't keep pumping out full power flash pops, and it has to give in somehow, so it pumps out partial power flash pops.

---Bob Gross---

[scottbergerphoto]

High Speed Sync actually reduces the freezing motion ability of the flash by reducing it's motion stopping ability to that of the shutter speed. You gain nothing in that regard. You also lose distance by virtue of reduced flash output. It is mainly used for flash fill on bright days.
Regards,
Scott

[timmyquest]

Thats pretty much what i was expecting.

*Tally another one up for the 1D*