800D wireless flash with high speed sync

Tourlou wrote:

Only for the slower exposure timings does the shutter fully opens. Let's say, to make easy numbers, that the shutter leafs can travel the focal plane in 1/500 second, which is ball park figure actual cameras. When you expose a picture at 1/200 of a second, it means that the front curtain goes down in 1/500 (.002 second) the system waits 3/1000 (.003 second) and then the rear curtain comes down 1/500 (0.002 second). Total exposure time is then (0.002+0.003=0.005 second (1/200). But the shutter has only been fully open for 0.003 second, which is the time you have to pop your flash. Otherwise, the front curtain or the rear curtain would blind part of the sensor.

For exposure times shorter than 1/500, the front curtain starts its travel and the rear curtain follows. This causes the sensor to be blinded by a part of the front curtain or the rear curtain at all time. For a 1/1000 exposure, half of the sensor is always blinded. Using a flash that has a persistance of less than 1/500 of a second will cause the picture to have a dark band on top or at the bottom. Most IGBT switched flashes have a 1/1000 discharge time at 100%, which disqualifies them for use above the speed of 1/250 or so.

WOOPS, sorry, I didn't see you're talking about high speed sync. In that case, the flash transforms to a high speed strobe with a very high switching frequency (IGBT allow for that) allowing to spread the lighting over the time it takes to the shutter to cross in front of the sensor.

It depends what you are shooting and what the ambient light is like. If there isn't much ambient light and you are using flash for all/most of the exposure you can set the shutter to say 1 sec and use the flash for all of the exposure. It doesn't matter then when the flash goes off during the 1 second open shutter. The rest of the second will not be recording anything.

The HSS shutter thing you describe is well explained here in a diagram. https://www.exposureguide.com/high-speed-sync-flash/ item #3