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Forums Lobby MASTER YOUR TOOLS Nikon Speedlights & Lighting topic #34874
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Subject: "CLS i-TTL Power Settings Independent of Group" Previous topic | Next topic
bclaff Silver Member Awarded for multiple contributions for the Resources   Vancouver (WA USA not Canada), US  Nikonian since 26th Oct 2004 Fri 12-Sep-08 12:39 AM
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"CLS i-TTL Power Settings Independent of Group"



Hal's (HBB) recent post reminded me to perform this test.

I shot 4 images of the same target with two SB-800s in TTL mode using an SU-800.
One SB-800 was in Group A. The other was in Group B.
Here are the resulting images, identically processed.



Upper left is using Group A and Group B
Upper right is using Group A
Lower left is using Group B
Lower right is using Group A and Group B but Group B was not pointed at the subject

If it isn't clear from the images, here are the histograms.



Clearly when Group B did not point at the subject; Group A was not increased in power to compensate.

I had hoped that the Nikon system was smarter than this.
That it would do all the pre-flashes and then compute all the powers.
But this test appears to confirm that power levels are set on each Group based on it's pre-flash (for i-TTL).

The practical implication would seem to be that is that it's unwise to illuminate the same portion of a subject with flashes from different Groups as their interaction is not taken into account.
(Typically, when I use more than one Group, I'm using one for "foreground" and another for "background"; and so I have never run afoul of this issue myself.)


Bill

Visit me at My site

Attachment #1, (jpg file)
Attachment #2, (jpg file)

  

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Replies to this topic
Subject Author Message Date ID
Reply message RE: CLS i-TTL Power Settings Independent of Group nl
12th Sep 2008
1
Reply message RE: CLS i-TTL Power Settings Independent of Group bclaff Silver Member Awarded for multiple contributions for the Resources
12th Sep 2008
2
Reply message RE: CLS i-TTL Power Settings Independent of Group Arkayem Moderator Awarded for his high level skills in flash photography
12th Sep 2008
9
     Reply message RE: CLS i-TTL Power Settings Independent of Group nl
13th Sep 2008
10
Reply message RE: CLS i-TTL Power Settings Independent of Group HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources.
12th Sep 2008
3
Reply message RE: CLS i-TTL Power Settings Independent of Group nl
12th Sep 2008
4
     Reply message RE: CLS i-TTL Power Settings Independent of Group HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources.
12th Sep 2008
8
Reply message RE: CLS i-TTL Power Settings Independent of Group Arkayem Moderator Awarded for his high level skills in flash photography
12th Sep 2008
5
Reply message RE: CLS i-TTL Power Settings Independent of Group bclaff Silver Member Awarded for multiple contributions for the Resources
12th Sep 2008
6
     Reply message RE: CLS i-TTL Power Settings Independent of Group Arkayem Moderator Awarded for his high level skills in flash photography
12th Sep 2008
7

nl   West Hartford, US  Basic Member Fri 12-Sep-08 01:48 AM
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#1. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 0



This is interesting / important data. I am NOT surprised at your results, actually, as this is exactly what I would have expected and _is_ what I would want the CLS system to do.

Image that you are using a non-CLS studio strobe setup. You set your ISO and desired aperture on your camera. You set up your key light and using your flash meter or whatever mechanism you are comfortable with, adjust the key light for the desired exposure.

Now, suppose you bring in a second light as fill. You would set the fill to the appropriate output for whatever lighting ratio you wanted to have. You would _NOT_ reduce the output of the key because you have brought in fill; the latter is used to bring additional illumination to an area of the subject not adequately lit by the key, not to allow you to reduce the key output.

I would expect CLS to work the same way, and so would expect each group to meter independently based on lighting ratios that you set up in the commander.

Therefore, if you set one flash in each of group A and B at 0ev compensation on the commander, I would expect if you put the two flashes next to each other, you would double the light on the subject from that position.

In contrast, if you put two remotes together in the SAME group, then the output of each flash will be 1/2; that is the key to doubling up the flashes in a give group to shorten recycle times, or alternatively to increase total output if the output of one flash is insufficient to properly exposure the image.

nl

  

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bclaff Silver Member Awarded for multiple contributions for the Resources   Vancouver (WA USA not Canada), US  Nikonian since 26th Oct 2004 Fri 12-Sep-08 02:08 AM
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#2. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 1
Fri 12-Sep-08 02:30 AM by bclaff


Arg... I need to sleep.

Visit me at My site

  

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Arkayem Moderator Awarded for his high level skills in flash photography   Richmond Hill, GA (Savannah), US  Charter Member Fri 12-Sep-08 09:49 PM
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#9. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 1



>This is interesting / important data. I am NOT surprised at
>your results, actually, as this is exactly what I would have
>expected and _is_ what I would want the CLS system to do.

Your comments make a lot of sense to me. I see now that I wouldn't want the system to compensate for one group from another group; at least not for shooting portraits where I want to set a particular lighting ratio using TTL and flash compensation.

Russ
http://russmacdonald.smugmug.com/
http://NikonCLSPracticalGuide.blogspot.com/

  

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nl   West Hartford, US  Basic Member Sat 13-Sep-08 12:50 AM
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#10. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 9



Thanks. I'm glad to see that my hypothesis is not way off base!

nl

  

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HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources.   Phoenix, US  Charter Member Fri 12-Sep-08 02:59 AM
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#3. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 0



Bill and Neal:

Bill: Nice piece of work!

This raises a question that I don't believe we have discussed yet: Do either of you suspect that the on-camera master/commander has any idea just how many remotes may be configured in each remote group?

I suspect not. With nothing more to go on than my preflash sequence image, I suspect that CLS measures the reflected light from the subject(s) group by group and simply returns a calculated exposure compensation value to the group as a whole. Overlapping illlumination is up to the user to detect and deal with.

One way to test this would be to repeat my setup with two SB800s in each of the three groups and see what, if anything, changes.

It would be really nice if the on-camera master/commander would enter the speedlight exposure compensation values for the on-camera master and each of the three possible remote groups in the XIF file. I was hoping for this in the SB900. No joy!

I also suspect that the CLS system does not account for overlappiing illumination from multiple groups. This would imply a final set of values to the remote groups following sampliing of all three. I don't see any evidence of this.

A final question for both of you. We know that newer cameras can collect subject to lens distance data with appropriate lenses. Do either of you suspect that the preflash sequence can collect speedlight to lens distance data? If yes, how would it handle multiple remote speedlight units some distance apart configured in the same group? Doubtful, me thinks, unless the preflash pulses are far more sophisticated than we think they are and every SB800 has a unique identity that is conveyed to the on-camera master/commander at some point. My test with multiple speedlights in each group may shed some light on this possibility.

Regards,

HBB in Phoenix, Arizona
Nikonian Team Member

Photography is a journey with no conceivable destination.

Join Nikonians at Photokina: Cologne, Germany
September 23 - 28, 2008: Hall 2.1, Booth E014

  

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nl   West Hartford, US  Basic Member Fri 12-Sep-08 03:49 AM
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#4. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 3



Hal:

I do not think that the master has information on how many remotes are configured per group. The only way I can think of for the master two know that would be for each remote to send some a signal back to the master, which could certainly be done by encoding something in the remote's preflash, but again I think this would require each flash to have a unique ID code. It could not be done via the intensity of the light metered on the preflash, because to know that you would have to know the reflectivity of the subject and the flash to subject distance.

I also do not think the master knows remote to lens or remote to subject distance. The only way you can know that is via transmission time between units, and the at the short distances we are working with, I cannot believe that the speedlights incorporate a timer capable of that kind of time resolution.

nl

  

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HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources.   Phoenix, US  Charter Member Fri 12-Sep-08 04:27 PM
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#8. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 4



Neal et al:

Neal: we are in agreement, as usual, re CLS sophistication.

Much of my rambling on CLS is motivated by my personal "wish list" for a next generation speedlight. I was hoping the SB900 would be a step or two in this direction ... not this time.

Russ: I too use different remote groups for different groups of subjects during my night shoots with law enforcement agencies. One group might be several officers in one area, talking about a crime scene. Another group in the same shot might be standing beside a patrol car. Yet another group might be a K9 with handler. This is why I was hoping for a few more remote groups: six would be nice.

For those of us deep into the CLS technology, and also deep into the financing of our herds, I suspect we could come up with the specs for the "Dream" speedlight system. Having Nikon produce it at an affordable price is another issue. We can dream, can't we?

Regards to all,

HBB in Phoenix, Arizona
Nikonian Team Member

Photography is a journey with no conceivable destination.

Join Nikonians at Photokina: Cologne, Germany
September 23 - 28, 2008: Hall 2.1, Booth E014

  

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Arkayem Moderator Awarded for his high level skills in flash photography   Richmond Hill, GA (Savannah), US  Charter Member Fri 12-Sep-08 12:16 PM
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#5. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 0
Fri 12-Sep-08 12:25 PM by Arkayem


Very interesting! I had always thought the system was smarter than that as well, although I have noticed many times that when shooting with the flash in remote mode, it always underexposes compared to using the flash on a SC-29 cable. Maybe that is an attempt by Nikon to guardband against overexposure when multiple flashes are used.

I guess I have never run afoul of this either, probably because I always use each flash for something different, usually Key, Fill, and Hair.

Russ
http://russmacdonald.smugmug.com/
http://NikonCLSPracticalGuide.blogspot.com/

  

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bclaff Silver Member Awarded for multiple contributions for the Resources   Vancouver (WA USA not Canada), US  Nikonian since 26th Oct 2004 Fri 12-Sep-08 01:49 PM
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#6. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 5



Russ,

I have noticed many times that when shooting with the flash in remote mode, it always underexposes compared to using the flash on a SC-29 cable.

I am not surprised.
When on the SC-29 cable it still "thinks" it is in the hot-shoe.
Then I imagine any distance information confuses the exposure system.
(I'm guessing that on the SC-29 the flash is closer to the subject than the hot-shoe.)

Regards,
Bill

Visit me at My site

  

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Arkayem Moderator Awarded for his high level skills in flash photography   Richmond Hill, GA (Savannah), US  Charter Member Fri 12-Sep-08 04:22 PM
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#7. "RE: CLS i-TTL Power Settings Independent of Group"
In response to Reply # 6



>Russ,
>
>I have noticed many times that when
>shooting with the flash in remote mode, it always underexposes
>compared to using the flash on a SC-29 cable.

>
>I am not surprised.
>When on the SC-29 cable it still "thinks" it is in
>the hot-shoe.
>Then I imagine any distance information confuses the exposure
>system.
>(I'm guessing that on the SC-29 the flash is closer to the
>subject than the hot-shoe.)
>
>Regards,
>Bill
>

I know that distance is used in the flash power computation when using TTL-BL mode with the head of the flash pointed directly forward (either in the hot shoe or attached via SC-29). In fact, you have to be very careful when using TTL-BL mode on the SC-29 cable, to keep the distance from the flash to the subject equal to the distance from the camera to the subject or you will not get proper exposure.

However, I can find no indication that distance is ever used when in regular TTL (or Remote) mode.

Regards,

Russ
http://russmacdonald.smugmug.com/
http://NikonCLSPracticalGuide.blogspot.com/

  

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Forums Lobby MASTER YOUR TOOLS Nikon Speedlights & Lighting topic #34874 Previous topic | Next topic