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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 22 Aug 2011 (Monday) 05:33
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which will consume more flash power?

 
ans3288
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Aug 22, 2011 05:33 |  #1

hi guys,

just thinking aloud, i have pw tt1 and tt5s now if i have a theoretical setting of:

1/4000 F2.0 ISO 200
1/2000 F2.0 ISO 100

since HSS happens by shooting multiple blasts of lower powered flash to the subject, which will be consuming more flash power?

yes, iso 200 will be more sensitive to light, but the shutter speed is double so will the PWs work harder by firing more often? can anybody help me out?

:)


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PacAce
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Aug 22, 2011 07:36 |  #2

Just thinking out loud here. Let's say that the light output at 1/2000 @ ISO 100 = X. If we double the shutter speed to 1/4000, with ISO still at 100, then we would need twice as mich light for the same exposure, i.e. at 1/4000 @ ISO 100, light ouput = 2X.

Bumping the ISO to 200 with the shutter kept at 1/4000 would double the exposure so the light output would need to be half of what it was at ISO 100. So, at 1/4000 @ ISO 200, ligh output = 2X / 2 = X.

Since X is also the light output at 1/2000 @ ISO 100, I'd say that both setting,i.e. 1/4000 @ ISO 200 and 1/2000 @ ISO 100, will require the same amount of light.

Edit: I'm assuming we are talking about the flash being in E-TTL mode because if not, the question would be meaningless.


...Leo

  
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Gatorboy
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Aug 22, 2011 07:44 |  #3

I'd say ISO 100 would require more power since aperture controls flash exposure, not shutter speed.


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PacAce
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Aug 22, 2011 07:50 |  #4

Gatorboy wrote in post #12977088 (external link)
I'd say ISO 100 would require more power since aperture controls flash exposure, not shutter speed.

Remember, we are talking about HSS here where shutter speed does indeed influence flash exposure.


...Leo

  
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Curtis ­ N
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Aug 22, 2011 08:34 |  #5

Leo's analysis seems logical to me. But there's another way to figure this out.
Put the flash on the hotshoe, put it in HSS mode and set your camera's exposure settings.
Half-press the shutter button and take a look at the distance scale on the flash unit.
The setting that gives you greater distance range is the one that would consume less power at a given distance.


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clarence
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Aug 22, 2011 08:45 |  #6

It's not a linear trade-off. Here are a couple of additional points to factor in...

The manual for the 430EX (page 16) states "With High Speed Sync, the faster the shutter speed, the shorter the effective flash range will be".
http://downloads.canon​.com …amera/Speedlite​-430EX.pdf (external link)

The PW TT1/TT5 ControlTL power output is increased as the shutter speed is decreased...
http://www.pocketwizar​d.com …s/technology/fp​_sync_hss/ (external link)

IMAGE: http://www.pocketwizard.com/upload/photos/563Image2_489FP_Sync_4200_edited.jpg

Note that as shutter speed decreases, since the pulse duration decreases, the HSS light output approaches normal (non-HSS) flash power... at 1/4000", the flash output is close to full-power at 1/250".

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clarence
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Aug 22, 2011 08:53 |  #7

In summary, I think you'll get more light output from the 1/4000" ISO 200 combination.

Because the pulse power at 1/4000" (magenta line) is higher than the level at 1/2000" (the gray plateau right below the magenta trace).


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tkbslc
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Aug 22, 2011 08:56 |  #8

Higher power at 1/4000, but the duration is shorter. So is it really more total power?


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PacAce
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Aug 22, 2011 09:15 |  #9

clarence wrote in post #12977425 (external link)
In summary, I think you'll get more light output from the 1/4000" ISO 200 combination.

Because the pulse power at 1/4000" (magenta line) is higher than the level at 1/2000" (the gray plateau right below the magenta trace).

The amplitude of the light by itself does not necessarily indicate the amount of light generated by a flash. You also need to take into consideration the duration of the light as well, as pointed out by tkbslc.


...Leo

  
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clarence
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Aug 22, 2011 11:44 |  #10

tkbslc wrote in post #12977447 (external link)
Higher power at 1/4000, but the duration is shorter. So is it really more total power?

PacAce wrote in post #12977562 (external link)
The amplitude of the light by itself does not necessarily indicate the amount of light generated by a flash. You also need to take into consideration the duration of the light as well, as pointed out by tkbslc.

Duration being shorter is balanced by the ISO being higher.

One stop of SS = one stop of ISO.

Thus, with all other things being equal, higher output power should yield more light power.


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MrScott
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Aug 22, 2011 11:54 |  #11

So isn't anyone going to recommend taking a pic and seeing average recycle times to determine most energy used? The one to rec quickest uses less power right????? Assuming both shots have equil exposure.




  
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PacAce
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Aug 22, 2011 11:57 |  #12

clarence wrote in post #12978431 (external link)
Duration being shorter is balanced by the ISO being higher.

One stop of SS = one stop of ISO.

Thus, with all other things being equal, higher output power should yield more light power.

I'm pretty sure that the graphs you showed in your post were all for the same ISO.

Besides, higher shutter speeds mean less area of the sensor being exposed. So, output needs to be higher to compensate for the decreased coverage of the sensor.


...Leo

  
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clarence
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Aug 22, 2011 13:42 |  #13

PacAce wrote in post #12978479 (external link)
I'm pretty sure that the graphs you showed in your post were all for the same ISO.

Read the original post...

1/4000 F2.0 ISO 200
1/2000 F2.0 ISO 100

So half the shutter speed and double the ISO cancel each other out.

The black graph is independent of ISO, showing different power outputs as the SS decreases.

Thus, more light output from the TT1/TT5 at faster shutter speeds.

PacAce wrote in post #12978479 (external link)
Besides, higher shutter speeds mean less area of the sensor being exposed. So, output needs to be higher to compensate for the decreased coverage of the sensor.

Read the original post...

1/4000 F2.0 ISO 200
1/2000 F2.0 ISO 100

So half the shutter speed and double the ISO cancel each other out.

The sensor has the same area exposed regardless of SS.


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clarence
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Aug 22, 2011 13:45 |  #14

tkbslc wrote in post #12977447 (external link)
Higher power at 1/4000, but the duration is shorter. So is it really more total power?

Duration is shorter, but ISO sensitivity is greater, so exposure is balanced.

Output power is greater at shorter SS (see graph), so more light capacity is available.


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Aug 22, 2011 15:15 |  #15

MrScott wrote in post #12978468 (external link)
So isn't anyone going to recommend taking a pic and seeing average recycle times to determine most energy used? The one to rec quickest uses less power right?? Assuming both shots have equil exposure.

Just did evaluation of my Metz with HSS enabled, mounted on a 40D, and using max distance readout (same as CurtinN's suggestion earlier) rather than timing recycle duration...

Max distance with HSS, ISO 100, 1/1000 f/2.8 = 6.4' according to the Metz distance display.
Max distance with HSS, ISO 200, 1/2000 f/2.8 = 6.4' according to the Metz distance display.
Max distance with HSS, ISO 100, 1/2000 f/2.8 = 4.5' according to the Metz distance display.
Max distance with HSS, ISO 200, 1/4000 f/2.8 = 4.5' according to the Metz distance display.

NO DIFFERENCE in the max effective range of the flash, when doubling ISO and adjusting shutter speed by 0.5x

And just to prove the Metz display of distance is meaningful...
Max distance with HSS, ISO 200, 1/1000 f/2.8 = 9.1' (1.4x farther than 6.4') according to the Metz distance display.


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which will consume more flash power?
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