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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 06 Apr 2018 (Friday) 03:57
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When to use HSS

 
Wilt
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Post edited 20 days ago by Wilt. (3 edits in all)
     
Apr 06, 2018 13:48 |  #16

dpe wrote in post #18601132 (external link)
Was trying to explain a rationale for using or not using HSS to some people I was demonstrating to, so put it into a flow diagram
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forum: Flash and Studio Lighting

Some facts that the chart does not factor in at the very first decision point, about 'freezing action'...I know, nitpicking but important for a novice to understand

  • If shooting with normal ETTL, the flash freezes action really only when at 1/2 power (or lower) when its duration of light emitted is <1/1000...full power flash is actually fairly slow (depending upon flash model) at 1/400 or 1/500
  • When in HSS, the shutter does the freezing of action only when set to <1/1000 (1/250 for FF shutter shifting to HSS flash is not very action freezing)


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Wilt
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Post edited 20 days ago by Wilt. (5 edits in all)
     
Apr 06, 2018 13:53 as a reply to  @ post 18601414 |  #17

dps wrote:
Are you sure that -3 EV is 1/3 distance? inverse square law would be 1/8 power or 1/8 distance
and -2EV would be 1/4 distance

Assume normal GN 100 flash, at f/4 light reaches 25'

  • -1EV results in GN70.7, so at f/4 flash reaches 17.7' (or 25' at f/2.8)
  • -2EV results in GN50, so at f/4 flash reaches 12.5' (or 25' at f/2)
  • -3EV results in GN35.3, so at f/4 flash reaches 8.8' which is about 1/3 of 25' (or 25' at f/1.4)
  • -4EV results in GN25, so at f/4 flash reaches 6.25' (or 25' at f/1.0)

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dpe
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Apr 06, 2018 13:53 |  #18

Wilt wrote in post #18601413 (external link)
Some facts that the chart does not factor in at the very first decision point, about 'freezing action'...I know, nitpicking but important for a novice to understand

  • If shooting with normal ETTL, the flash freezes action really only when at 1/2 power (or lower) when its duration of light emitted is <1/1000...full power flash is actually fairly slow (depending upon flash model) at 1/400 or 1/500
  • When in HSS, the shutter does the freezing of action only when set to <1/1000 (1/250 for FF shutter shifting to HSS flash is not very action freezing)


Yep that is real nitpicking because there is movement that will be frozen at 1/100s and other that will not be frozen at 1/1000s - alreadty decided that needs updating to include reference to shooting above x-sync

Mike


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Wilt
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Post edited 20 days ago by Wilt. (2 edits in all)
     
Apr 06, 2018 14:03 |  #19

dpe wrote in post #18601416 (external link)
Yep that is real nitpicking because there is movement that will be frozen at 1/100s and other that will not be frozen at 1/1000s - alreadty decided that needs updating to include reference to shooting above x-sync

Mike

Actually, my own version of your chart would have been much simpler, to ask ONLY "Do you need to shoot with shutter faster than X-sync", and the question about maximizing the ratio of ambient:flash intensity (using HSS makes the ratio larger)
The choice of aperture (for light gathering and DOF control) has nothing to do directly with shutter speed (since you can regulate the corresponding shutter speed via ISO selection)

The balance of your decision tree is fundamentally the same for ambient-only as it is for ambient+flash, which is why it could be left out.


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Apr 06, 2018 14:09 |  #20

Wilt wrote in post #18601424 (external link)
Actually, my own version of your chart would have been much simpler, to ask ONLY "Do you need to shoot with shutter faster than X-sync".
The choice of aperture (for light gathering and DOF control) has nothing to do directly with shutter speed (since you can regulate the corresponding shutter speed via ISO selection)

Except that cameras are limited by physics or better said their sensor, ideally I would want a sensor that goes down to 5 or even 1 ISO but they do not exist so if you want f1.4 in the mid day sun then you need to reduce ambient, staying below X-sync gives a better ration of flash to ambient than being in HSS mode, so using ND filters is a very good solution

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Wilt
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Post edited 20 days ago by Wilt.
     
Apr 06, 2018 14:13 |  #21

dpe wrote in post #18601430 (external link)
Except that cameras are limited by physics or better said their sensor, ideally I would want a sensor that goes down to 5 or even 1 ISO but they do not exist so if you want f1.4 in the mid day sun then you need to reduce ambient, staying below X-sync gives a better ration of flash to ambient than being in HSS mode, so using ND filters is a very good solution

Mike

A few moments ago I did already alter my earlier reply to show that the decision tree about ambient:flash ratio being very valid consideration that I would include in my own version of the decision chart.


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Apr 07, 2018 14:28 |  #22

Wilt wrote in post #18601407 (external link)
Typically flash units lose at least -2EV in power when shifting into HSS light output, meaning that you have to use 2EV faster aperture, or your flash distance is cut in half.
AND THEN, for every 1EV faster shutter speed, the reach of the flash is cut by another -1EV, so you could lose a total of (at least) -6EV if needed to shoot at 1/5120 instead of 1/200 (for a FF dSLR) -- or about 1/8 of the usual distance achievable at any particular f/stop.

Some flashes start with losing -3EV, or the flash distance is cut to 1/3 its usual distance! AND THEN, for every 1EV faster shutter speed, the reach of the flash is cut by another -1EV,

I am always taken aback when people say that HSS cuts the flash power.
What HSS does is spread the power in the capacitor over the duration of the shutter curtain travel across the sensor.
Its a distribution of power over a longer time than the peak and fade distribution for normal x sync flash duration.

Like any constant light source, when the shutter speed increases, shortening exposure and narrowing the slit between leading and trailing curtains, there is less time for the light to energize the sensor. That's not the flash losing power, its the exposure triangle operating as expected with a constant light source.


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Apr 07, 2018 17:32 |  #23

dmward wrote in post #18602002 (external link)
I am always taken aback when people say that HSS cuts the flash power.
What HSS does is spread the power in the capacitor over the duration of the shutter curtain travel across the sensor.
Its a distribution of power over a longer time than the peak and fade distribution for normal x sync flash duration.

Like any constant light source, when the shutter speed increases, shortening exposure and narrowing the slit between leading and trailing curtains, there is less time for the light to energize the sensor. That's not the flash losing power, its the exposure triangle operating as expected with a constant light source.

HSS “power drop” versus HS no power drop. ??


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Post edited 18 days ago by Wilt. (5 edits in all)
     
Apr 07, 2018 18:17 |  #24

dmward wrote in post #18602002 (external link)
I am always taken aback when people say that HSS cuts the flash power.
What HSS does is spread the power in the capacitor over the duration of the shutter curtain travel across the sensor.
Its a distribution of power over a longer time than the peak and fade distribution for normal x sync flash duration.

Like any constant light source, when the shutter speed increases, shortening exposure and narrowing the slit between leading and trailing curtains, there is less time for the light to energize the sensor. That's not the flash losing power, its the exposure triangle operating as expected with a constant light source.

it is true that the amount of 'electrical power' is the same whether in HSS or not....there is a single capacitor used for both modes, and it has a finite amount of electrical power.
However, the light output power is indeed diminished by -2EV or -3EV (the amount dependent upon flash, even two units of the same model are known to vary from one another!), so with regard to useful light intensity there is indisputably a POWER DROP due to HSS. And that is why the range indicator on the back LCD of a Canon 580EX diminishes in distance, as the light intensity has diminished so it cannot reach as far at the identical f/stop.


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Post edited 18 days ago by Methodical. (2 edits in all)
     
Apr 08, 2018 11:23 |  #25

Sibil wrote in post #18601256 (external link)
Good idea. I have never used HSS flash, and this should be useful to get started. I am thinking bird photography with bright sky backgrounds.


dpe wrote in post #18601265 (external link)
Just remember the inverse square law...Mike


Sibil wrote in post #18601317 (external link)
Got it. Thanks.

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Apr 08, 2018 12:36 |  #26

Sometimes, when on location the background is not ideal and you want to eliminate certain distracting elements. Those times, HSS is another tool that is available to either darken the background or open up the lens.


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Apr 08, 2018 13:37 |  #27

Methodical wrote in post #18602408 (external link)
Sibil get yourself a Better Beam flash Extender (external link)

Thanks. It is on my list.




  
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Apr 11, 2018 11:49 |  #28
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Wilt wrote in post #18601407 (external link)
Typically flash units lose at least -2EV in power when shifting into HSS light output, meaning that you have to use 2EV faster aperture, or your flash distance is cut in half.
AND THEN, for every 1EV faster shutter speed, the reach of the flash is cut by another -1EV, so you could lose a total of (at least) -6EV if needed to shoot at 1/5120 instead of 1/200 (for a FF dSLR) -- or about 1/8 of the usual distance achievable at any particular f/stop.

Some flashes start with losing -3EV, or the flash distance is cut to 1/3 its usual distance! AND THEN, for every 1EV faster shutter speed, the reach of the flash is cut by another -1EV,


Yeah..with HSS, you lose at least 2 stops of light. If the shooting distance is short, that's not a problem. The cycling time is cut short too and the number of bursts the flash would fire is inconsistent. You may get 3 frames in a row or less.....even with a battery pack. For this reason, I will go with hyper sync or super sync....:) Not only your shutter speed will freeze action, the flash power output won't get cut at all....


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