1/250 rule with OCF?

http://digitalprotalk.​blogspot.com …-my-cheatin-light-or.html

HERE is a good explanation of HSS, he's using canon speedlights and PW triggers, but you can do this with a 580 on board, the 7D's wireless flash control, or the ST-E2. You can also use the flashes in manual or ETTL.

All well and good but you can see how close the modifier is to the subject to make that happen. If you try to use a Speedlight in high speed sync and want to shoot full length with some breathing room in the frame you find out quickly that the flash chokes and can't give you the output needed.

High speed sync works great in certain scenarios but it's not something you can count on all the time. It has its limitations.

One quick way to determine the impact of the shutter speed on deminishing flash power with a Canon 580EX is to look at the effective exposure distance scale on the back of the flash. Lower the shutter speed to 1/200 (or whatever your camera's sync speed is) and look at the distance scale for the F stop and zoom you have set. Now increase the shutter speed to the where you need it and the distance scale will show you how close you have to be for HSS to properly expose the subject.

The thing you quickly notice is that the flash has to be close.

My 580EXII in a 24" softbox works fine in HSS for head and shoulders. It would be marginal at half length and probably not usable for full length.