High speed sync issues and observations

I ran in to a few issues with High Speed Sync, and wanted to document them for further reference.

First, though the Camera and flash manual say that you can use wireless High Speed Sync with the built in flash as a commander, this appears to be untrue. You can only use wireless high speed sync if you have a second 360 or 540 flash. So if you need high speed sync, for now you'll need to keep the flash on the camera if you need fill light.

Secondly, every time we had mentioned that flash duration can be shorter than 1/1000th of a second, so the aperture will affect the ambient light and the flash, while the shutter speed generally only has an effect on the ambient light as the flash duration is too short. However, when you use high speed sync, that is not the case.







Ordinarily, the sensor is covered by two "curtains". One moves to reveal the sensor, and the second moves to cover the sensor. When the exposure is too short (faster than 1/180th for the Pentax K10d), the second starts to move before the first has cleared the sensor, so there is no one time where the flash can light the entire sensor with one burst. So High Speed sync shoots off the flash in many short bursts, covering the entire sensor as the slit between the two curtains moves across the sensor.

So for exposures faster than the high speed sync speed, the subject to be lit does not receive as much flash energy on a shorter exposure because there is less time to shoot as many bursts. Above are photos taken at 1/180, 1/250, 1/500, 1/1000, 1/1500, 1/2000, 1/3000, and 1/4000th of a second. The amount of light from the flash decreases for each one.