More dumb HSS questions

[jeffbox]

I'm trying to better understand the relationship between shutter speed, flash, and exposure.

Keeping iso 100, f/8, and flash power (1/2 on a 430ex) constant, I changed the shutter speed. At 1/60, 1/180, and 1/200 the results looked pretty much the same (proper exposure). When I went to 1/250 (HSS territory for my Xsi,) it was under exposed and the flash power seemed weaker. Then I went to 1/500 and it was even weaker. At 1/1000 you could barely see anything. I understand HSS takes more juice and it's weaker, but why did my picture change from 1/250 to 1/500 to 1/1000 if they were all HSS?

My only guess is that there was still some ambient light? My room was pretty dark. A correct nonflash exposure wants me at around 30 seconds to get the right exposure.

thanks for the help

[alphonsis]

I forget where it is, but there's a good animation which explains HSS. You can try googling for it if my explanation falls short.

Normally, your flash shoots one bright burst of light to expose your shot. Once you're in HSS territory, the two curtains covering your sensor move so fast, that the full duration of your flash can't happen while the whole sensor is exposed. As a result, the flash switches to shooting alot of lower power pulses while the curtains move across your sensor to give you an even exposure. As a result, the effective light output is diminished.

[Curtis N]

Keeping iso 100, f/8, and flash power (1/2 on a 430ex) constant, I changed the shutter speed.HSS isn't really "flash" in the traditional sense. For all practical purposes, HSS is a continuous light source that lasts throughout the duration of shutter curtain movement.

So increasing the shutter speed while keeping the flash power constant will result in decreased exposure, just as with any other continuous light source.

[jeffbox]

there's a good animation which explains HSS.

Maybe Stans video? That vid helped me a lot but I remember him saying "if your subject is lit primarily by flash, then you can increase the shutter speed and it will only affect the background" or something like that. I think I took that concept too far. I guess like Curtis said HSS is more like a continuous light source.

So increasing the shutter speed while keeping the flash power constant will result in decreased exposure, just as with any other continuous light source.

that makes a lot of sense, thanks.

[Wilt]

Because HSS is taking the same total energy and spreading out over a longer duration of time, there is less light at any period of the total time. (Think of a gallon bucket of water thrown at you all at once, vs. a gallon sprinkler bucket) With each -2EV of shutter speed you have a flash Guide Number that is reduced by half, as a result.