Yeah, it'll either shrink to a 430EX or grow to a 580EX.
Seriously, though, if you have HSS activated but shoot below max sync speed it'll automatically revert to normal mode, only going into HSS at high sync speeds.
If you're shooting above max sync a lot, let the flash cool down a bit between shots, as prolonged shooting can cause over heating -- or so I've been told.
A Canon flash in the hot shoe of a Canon camera communicates via the TTL pins. That can cause some unexpected surprises in exposure control. HHS is a tool. Like a hammer if you don't understand how best to control it you can hit your thumb and it will hurt. The "hurt" comes two ways with flash and sync limit: camera limiting shutter and overexposing ambient background and where flash overlaps it, and HHS kicking in unexpectedly and the photographer not realized its range is being exceed.
In normal flash mode you'll find that when shutter speed is set manually to say 1/1000th when you half-press your shutter you will see 1/250th (i.e., your sync limit) shown in the viewfinder and top LCD display on the camera. The camera knowing the flash is on will not let the shutter go above sync speed with a Canon flash. If you don't notice this and take the picture anyway the ambient background will be 2 stops over-exposed and the flash metering seeing that will not output a normal amount of flash in the foreground. More than one photographer has blamed the flash metering when this has occurred, but it's their bad for not having situational awareness of the lighting and camera settings.
Keeping the flash in HHS all the time allows the flash to switch to HHS mode when shutter crosses the sync-limit Rubicon. That will solve the problem of the camera locking shutter at 1/250th when you set 1/1000th and winding up with blown highlights and wondering why. When camera limits shutter to 1/250th it blows everything by 2 stops and the camera metering can't tell how much flash is really needed. In HHS the range of the flash is cut in half or more (depending on how much ambient is contributing) and beyond 5-6 ft. with a 430ex the foreground will probably be underexposed to some degree outdoors.
In Av mode or P where camera controls the shutter you must watch what shutter speed the camera has selected to know what your effective flash range will be. It's easy to loose track when shooting and miss the fact the shutter speed has wandered up about 1/250th. If you are aren't in the habit of "chimping" regularly and see the underexposed foreground you might not know the flash has switched to HHS and you are outside of it's effective range.
The most fool-proof way the use HSS is with camera in M mode with you controlling the shutter speed and ETTL controlling the flash. That way if flash is set for HHS mode you can control when it is used via how the shutter is set. If you are within the max range of the flash in HSS / ETTL mode the flash metering will adjust the flash exposure. If shooting in HHS with M mode flash you will need to manually adjust flash power for distance to obtain correct foreground exposure.
Tv mode will provide the same predictability as M for flash mode, but you loose control of DOF because the camera metering will decide the aperture.
I find Av mode with ETTL flash control the most convenient when using HSS outdoors. It allows me to move arpund from shade to sun with lens set wide for shallow DOF and less distracting backgrounds. I shoot into the shadows (facing sun) keep the sunny highlights below clipping with around - 2 EC (the backlight "fools" the ambient metering) and the camera settings wind up around "Sunny 16" adjusted for whatever aperture I've set at ISO 100:
From experience I know that outdoors Av setting wider than f/5.6 may cause the flash to flip-flop between HHS and normal as I move from sun-to-shade. If I'm shooting at f/4 or wider the flash will usually stay in HSS most of the time. Knowing that I stay within 8ft. of my flash illuminated foreground subject . Should the sun go behind a cloud and the ambient metering move the shutter to 1/250th or less causing the flash to switch back to normal, having the flash in ETTL mode will allow it to adjust flash exposure automatically.
In situations outdoors where I know I will be further than 7-8ft. and out of the range of HSS I shoot in M mode at 1/250th then adjust aperture as needed not based on the DOF I want (and usually can't get) but to keep sunlit highlights under clipping. The exposure winds up around 1/250th @ f9 - 11.
Try this: With your flash in the 0° tilt position and normal ETTL mode half press the shutter button and look at the back of the flash. See the bar graph? In ETTL it tells you the effective range for the flash (min and max distance), taking into account the ISO, aperture and zoom factor. Switch flash mode to M. The display changes to a single bar showing the distance where exposure is correct. Set power to 1/1 and the HHS button and see what happens.
Now change various camera settings: aperture/shutter/focal length....
For example set at 1/250th @ f/11 @ ISO 100 the display on my 580ex shows tells me at 70mm my ETTL range is 2-25 feet. But when I zoom the lens to 24mm and the flash head zooms with it and the flash range display changes to 2-12ft. Changing shutter to 1/320 forces it into HHS mode and the range indicator changes to 2-6ft. When direct flash is used the display will tell you the flash limits. It can't do that flash head is tilted up, and if using a modifier you need to test to know the limits.
A typical situation where I'd use HSS would be a portrait shot from 8ft at 70mm @ f/4 for a H&S shot similar to this one in backlit by sun as "hair light". http://super.nova.org/TP/BelenBacklightingE.jpg
Setting the shutter to keep sunny highlights under clipping @ f/4 would put me at around 1/1250. Zooming the lens to 70mm and half pressing the shutter the flash display tells me that in HHS @ 70mm @ f/4 @ 1/1250th my max direct flash range is 12ft. Since I use a bracket I can use direct flash and get flattering results. If I opt to use my DIY diffuser I have a dilemma. They cut flash power in half because they spread the light more resulting in about 1-stop less exposure on the face in direct flash. So for a "normally" exposed face with diffuser I'd need to move closer in HSS mode. But in backlight I don't need or want "normally" exposed white jacket and face, I want the flash lit side 2/3 stops below clipping. So things work out perfectly in that situation. Shooting in HHS with the diffuser to make the highlights less specular the jacket and face lit with the flash wind up about 1-stop below the sunny highlights — perceptually (i.e. looks "normal") correct exposure for a backlit subject see in person (not per any numerical targets).
With use and a bit of systematic testing you can also gain situational awareness of the limits of your gear with and without any diffusers and know how close you need to stay in HHS mode and when you need to switch to regular flash and deal with the greater DOF in other ways, such as in the shot above finding backgrounds that aren't full of distractions. I didn't pick that one by chance. The shot strategy was built around the need to put the white jacket on a similar light background to keep it from being distracting, hence the sunlit river behind her.
Start with the goals for the shot, then find the most effective strategy to make the focal point contrast, then pick the best tool for the job and practice so you know how to hit the nail squarely and not your thumb.
For anyone reading this who hasn't bought a flash yet, keep in mind that a 580exII with .8 stop more power will translate in to greater range in outdoor situations in HHS. Also using two flashes, overlapping the off axis "key" light over the fill on a bracket increases the effective range .
Great HSS description Chuck! I'm pretty sure I understood the majority of what you've described here. I'm such a visual and hands-on type learner that I feel the need to get out there and do just like you said and practice. If I wasn't leaving for Korea in a couple months I would definitely be registering for photography classes at CSN. Still working on your site...
