I was doing some testing yesterday with my PW MiniTT1 and 5DmkII and decided to see if I could calibrate the HyperSync Offset. What I found was that everything was good up to 1/250, as expected, but that for 1/320, 1/400, 1/500 I could not find an offset setting where I could create an image that didn't have either front (top of image) or rear (bottom of image) curtain clipping.

I think the default offset was -170. I noticed that at 1/320, the top of the frame was a little fuzzy (i.e. front curtain clipping), but that you could definitely see the shutter dragging at the rear. The supposed cure for this is to adjust your HyperSync offset to the left (larger number). When I tried this it did lessen the amount of rear curtain clipping, but introduced more front curtain clipping. At 1/400 and 1/500, the effect was even worse and it didn't really matter what the HS offset setting was, as the net effect of changing its value was to just move the exposed portion of the frame (i.e. part that was lit between the shutter closing and opening) up and down on the image.

At 1/640, High Speed Sync (HSS) kicked in and all was good. Obviously, I could force HSS to kick in at a slower shutter speed, or it appears that this is an option in the PW utility, but I'd like to be able to extend the shutter speed range where I can get full power flash--1/320, 1/400, and 1/500--if possible.

Right now, it looks as though I'll have to shoot at x-sync speeds of 1/250, or manually push it to 1/640, or higher, and live with the lessened flash power that HSS affords, which is not all that bad.

In summary, I'm really just looking to see if anyone was able to find a HyperSync offset setting that fixed the curtain clipping issue for the three shutters speeds that I mentioned, using the MiniTT1 and the 5DmkII?

After looking at the Strobist threads and the PW forum on Flickr, it appears as though this is a unique situation with the 5DmkII. It seems as though, in the past, HSS wasn't enabled until you reached 1/640, but that in the latest firmware release, the option to begin HSS earlier, i.e. 1/250 if you want, is enabled.

What I can't seem to find is if this issue is something that could be corrected with the proper firmware, or if it has something to do with the 5DmkII shutter design and will never be an option. I guess Radio Poppers wouldn't help if it's the latter...

Faking high speed sync works by firing the flash before the shutter opens then zipping the shutter curtain slit across the sensor within the duration of the single flash burst.

But what you need to realize about a focal plane shutter is that even when the shutter speed is set at 1/8000th the actual shutter curtains are traveling at the same speed they do at 1/8th. What changes the exposure is the gap between the two curtains created by the timing of how soon the second curtain starts to close after the first one opens. The x- speed of 1/250th on a camera reflects the time it takes for the first shutter curtain to clear the sensor completely, so we can infer that the actual curtain speed is around 1/300th sec.

What all that means with respect to faking high speed is that the flash duration needs to be long enough to give the flash time to come to full intensity and stay that way long enough for the curtain slit to move across the sensor at the relatively slow pace of 1/300th sec. Thyristor switched hot shoe flashes control power with flash duration so lower power settings will result in slower flash durations: not what is desired here. For this workaround longer is better. One strategy to maximize flash duration would be using the flash at full power to get the longest flash duration possible, regulating the power as needed with ND gells. Each .30 if ND added will cut flash output in half. .30 = 1/2 power, .60 = 1/4 power, etc.

All things considered its probably simpler to just use the built-in HHS mode as designed, allowing it to kick in when the shutter exceeds 1/250th. It works in a similar way. There is no "sync" between the pulsing of the flash and the shutter, the camera just fires up the flash at 40,000hz just before the shutter opens and keeps it pulsing long enough (1/300th sec) for the curtain slit to zip across the sensor. Here the camera treats the flash as a continuous source in the same way that the camera sees fluorescent lamps as continuous despite the fact they are pulsing at 2x the frequency of the current (120x per sec at 60Hz). That is why shutter speed affects exposure in high speed FP mode.

Radio Poppers, which simply relays the Canon commands with plug n' play simplicity, seem to be a more elegant solution to extending the range of Canon flash than the PW offering. Unfortunately the choice is solid product from flakey start-up or flakey product from an established one which I why I still just use what Canon gave us.

Chuck