HSS not working as advertised

please move this if it belongs somewhere else. i looked through all of the forums for "talk" regarding flash/strobes but i only saw talk about "gear". this is more about "why isnt this working"...

5D3, 600ex-RT off camera fired by ST-e3-RT...

Manual mode
1/200
f8.0
flash fired into reflector umbrella set on 1/4 power

gives proper exposure (portraits) and the white wall (background) is a perfect 18% gray

setting the flash (through the transmitter, thats what its designed to do) to High Speed Sync and cranking the ss up to 1/800 to give me a nice black background only underexposes the entire shot by 2 stops. even if i open the aperture to f4.0 in conjunction with the ss change (which i shouldnt need i dont THINK), i get underexposed images...

i must be missing out on something...

I can't follow what your set up is.

windpig wrote in post #17800019
I can't follow what your set up is.

camera is a 5d3,
flash (off camera) is a 600ex-rt
trigger (transmitter?) is an st-e3-rt

Ltdave wrote in post #17800026
camera is a 5d3,
flash (off camera) is a 600ex-rt
trigger (transmitter?) is an st-e3-rt

Just a thought. Is your ISO set to Auto?

I'm assuming you also upped the speedlite power when you went to 1/800? Or the ISO.
Using a reflector umbrella if 1/200 f/8 is 1/4 power I'd think you'd be at pretty much full power to get 1/800 at f/8, or maybe more.

HSS will need more power since the flash fires multiple times during the exposure to make sure it does not get blocked by the shutter.

You lose a lot of flash power with HSS, as noted above. It sounds like you want to maintain a normal exposure for your subject but drop the exposure for the background. HSS is probably not the answer here; it's mainly useful to freeze action or allow use of a wider aperture (see here for more - http://neilvn.com …igh-speed-flash-sync-hss/). One way to accomplish what you're trying to do is to modify your setup so the background is primarily lit by ambient light, not the flash, then use camera exposure to darken the ambient while adjusting the flash output to properly expose the subject.

EDIT: Here's one way to do it - http://digital-photography-school.com …-with-light-in-portraits/
and here's another - http://petapixel.com …d-to-your-shots-anywhere/

HSS is a nice feature but you can't get around the limitations of max sync speed without costs. The conversion from a single burst to a series of pulses costs well over a stop of power and then gets greater as your shutter speed increases. In most applications a single HSS flash with a modifier is not enough. They are fine bare for fill but not very effective for a key light in a modifier.

jay125 wrote in post #17800037
Just a thought. Is your ISO set to Auto?

nope...

TerminalCity wrote in post #17800050
I'm assuming you also upped the speedlite power when you went to 1/800? Or the ISO.
Using a reflector umbrella if 1/200 f/8 is 1/4 power I'd think you'd be at pretty much full power to get 1/800 at f/8, or maybe more.

im just learning to use fractional power on my flashes (ive watched several videos about doing so, to save battery and get faster recycle. so i kept the flash at 1/4 power

Strick wrote in post #17800248
HSS will need more power since the flash fires multiple times during the exposure to make sure it does not get blocked by the shutter.

stsva wrote in post #17800260
You lose a lot of flash power with HSS, as noted above. It sounds like you want to maintain a normal exposure for your subject but drop the exposure for the background. HSS is probably not the answer here; it's mainly useful to freeze action or allow use of a wider aperture (see here for more - http://neilvn.com …igh-speed-flash-sync-hss/). One way to accomplish what you're trying to do is to modify your setup so the background is primarily lit by ambient light, not the flash, then use camera exposure to darken the ambient while adjusting the flash output to properly expose the subject.

EDIT: Here's one way to do it - http://digital-photography-school.com …-with-light-in-portraits/
and here's another - http://petapixel.com …d-to-your-shots-anywhere/

gonzogolf wrote in post #17800277
HSS is a nice feature but you can't get around the limitations of max sync speed without costs. The conversion from a single burst to a series of pulses costs well over a stop of power and then gets greater as your shutter speed increases. In most applications a single HSS flash with a modifier is not enough. They are fine bare for fill but not very effective for a key light in a modifier.

yep. see reply above. i am just learning this whole flash thing and like i said, didnt really know i needed more flash power.

my background IS lit only by ambient and it wasnt much at that. ive got more figuring to do...

thanks

Consider this...subject at 4', background at 5.6', fire any flash into that scene and when the subject is perfectly exposed, the background will be -1EV darker. So then we turn on HSS...

1. At shutter speed 1/200 with f/ 8 on normal lens (5D3), GN=138 at full power reaches to 17'...in normal flash mode, the flash needs to output at 1/4 power to achieve correct exposure at f/8 at 4' subject distance.
   
2. At shutter speed 1/250 with f/ 8 on normal lens, GN=69 at HSS full power reaches to 8.5'...in HSS flash mode, the flash needs to output at HSS 1/2 power to achieve correct exposure at f/8 at 4' subject distance. Subject is at 4' properly exposed at f/8, background is -1EV at 5.6'.
   
3. At shutter speed 1/500 with f/8 with normal lens, GN=34 at HSS full power reaches to 4.2'...in HSS flash mode, the flash needs to output at full power to achieve correct exposure at f/8 at 4' subject distance. Subject is at 4' properly exposed at f/8, background is -1EV at 5.6'.
   
4. At shutter speed 1/1000 with f/8 with normal lens, GN=17 at HSS full power reaches to 2.1'...in HSS flash mode, the flash needs to output at full power but -1EV underexposure at f/8 at 4' subject distance. Subject is at 4' at -1EV underexposed at f/8, background is -2EV at 5.6'.
   

Ltdave wrote in post #17800386
nope...

im just learning to use fractional power on my flashes (ive watched several videos about doing so, to save battery and get faster recycle. so i kept the flash at 1/4 power

yep. see reply above. i am just learning this whole flash thing and like i said, didnt really know i needed more flash power.

my background IS lit only by ambient and it wasnt much at that. ive got more figuring to do...

thanks

Hi, M mode on the camera for mixed ambient & flash scenarios is usually a good idea.

If the Speedlite(s) had been left in ETTL and operating under HSS, then the system would have tried to compensate for the HSS losses for you automatically - up to the point where they simply ran out of power headroom.
With the Speedlites set to Manual flash, then you have to adjust their power to allow for the HSS losses - typically 2.5 stops or more as the shutter speed rises further. HSS chews through Speedlite batteries at very roughly X5 the normal rate. Cheers

If you're trying to drop the background to black, as noted above HSS is probably not what you're after. Along with the other links above, you might find this video useful, Black Background: Ep 143: Exploring Photography with Mark Wallace https://youtu.be/YDzmi​Nl0j48.
He's using a strobe but it works the same with a speedlite.
Basically you need to:
a) avoid light spilling on the background
b) get your light closer to the subject

thanks everyone...

i think the first part of the issue, is that i had the subject too close to the background...


and Moderator, i THOUGHT about putting this in this forum originally but it wasnt so much about the GEAR, as it was my PROCESS. thanks for moving it though!

cheers all