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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 05 Feb 2014 (Wednesday) 15:30
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What is Gr Mode?

 
yogestee
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Feb 07, 2014 19:54 as a reply to  @ post 16668926 |  #16

Wrong flash sync. Grrrrrrrrrrrrr!


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apersson850
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Feb 08, 2014 13:28 as a reply to  @ yogestee's post |  #17

The old system allows you to set your slave flashes to belong to either group A, B or C.
If you then use E-TTL mode, you can use all slave groups together as one single source of light. The power will be set so that they combined contribute the light required for a standard exposure (per Canon's definition).
You can also use the A:B relation, where you weight the two groups to provide equal (1:1) or some different (like 4:1) ratio of light on the subject. Put together, they'll still render a standard exposure.
Then there's the A:B C mode, where groups A and B will be used as with the A:B setting, but then in addition the C group will be treated as an independent E-TTL group. Remember that the combined light from A and B will give the main subject a standard exposure, so if you let the light from the C group also illuminate the same subject, you'll get a one stop overexposure, since you have two E-TTL components illuminating the same target. Thus the C group must be directed to illuminate a background, which the main A and B groups can't reach. Or you'll have to play with individual flash exposure combination for the A:B and/or C groups.
Now in E-TTL, all flashes in groups A, B and C will be in E-TTL. (Yes, I know, individual slaves - let's leave them out of the equation now.) You can set everything up from the camera, but they are all E-TTL.

If you instead want to go to manual mode, then all slaves in all groups (A, B and C) will now be set manually. You can still set the power level for the three groups from the camera/master, but they are now all manual.

The new Group mode is different, in several ways.

  • There are five slave groups, not just three.
  • The mode of each group can be set individually (E-TTL, manual, Ext A or off).
  • There's no "use them all" mode.
  • There's no ratio mode (A:B).
Thus you can now mix E-TTL and manually set power levels, and you can change all this from your camera. You don't have to walk over to the slaves for anything but assigning group membership to them.
You can also individually turn off a group, if you for example want to look at what one group at a time will do to your picture, before you turn them all on and take the real shot.
However, since there's no ratio mode (X:Y), this means that if more than one group is set to E-TTL, they'll each provide enough light for a standard exposure. If you let four E-TTL groups hit the subject, then you get two stops of overexposure (four times as much light as you should have). You have to set the flash exposure compensation to appropriate negative values to handle this.
Most commonly the group mode would be used with most groups in manual mode, illuminating various fixed parts in a setup, then perhaps one E-TTL group illuminating some (main) subject, where the distance between flash and subject changes over time. As long as the manually set groups don't hit the main subject too, they'll not interfere with E-TTL metering, since each group is metered individually.

Note that the 600 EX-RT, the only Canon flash so far to support this new group mode, also fully supports the old system. The old system runs over both optical and radio communication, but group mode implies radio.

Anders

  
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Wavy ­ C
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Feb 08, 2014 16:48 |  #18

Anders, that's a very good answer above - a clear and informative description of how the various flash modes work.

My guess is that for situations like taking portraits, the best choices would be the three-group mode that allows for a main light (A), a fill light (B) and, if required, a background light (C). In ETTL, this setup allows for the ratio between the main and fill light to be adjusted as required and the system automatically looks after keeping the overall exposure correct. Or if you prefer manual then main, fill and background can all be set independently of each other.

The above is available on both the older flashes and the new 600ex system.

The 600ex adds the new GR or group mode.

A possible 'real world' situation for the GR mode would be to have an on-camera flash set to ettl and several other flashes around a room set to manual (as someone described above). If using more than one flash in ettl, a significant difference with the three-group mode (A:B + C) is that if the light from two or more flashes falls on a subject then the subject will be over-exposed. That is because in GR mode each flash operates independently of each other (so two or more on a subject will overexpose the subject). In the three-group mode the user can vary the ratio between two flashes but the system keeps the subject correctly exposed.

In manual mode, the new GR mode gives control over five flashes instead of three flashes on the previous system.



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Wavy ­ C
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Feb 12, 2014 10:03 |  #19

I've only got one 600ex and YN transmitter so haven't had much chance to play around with the GR mode (would need two flashes for that).

However I got a chance to use a friend's flash along with my own yesterday and was surprised to see that having two flashes set as slave A and slave B, and both pointing at the same subject, DOES NOT over-expose the subject.

I'd thought (as said in the previous post) that each flash operated independently when in GR mode. So, in ETTL mode, each flash would give the correct exposure for the subject. If there were two flashes pointing at the same subject that would mean the subject would get double the correct exposure. That does not appear to be the case.

As far as I know neither the YN manual or 600ex manual explains this in a clear way.



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dmward
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Feb 13, 2014 08:59 |  #20

Wavy C wrote in post #16674455 (external link)
A possible 'real world' situation for the GR mode would be to have an on-camera flash set to ettl and several other flashes around a room set to manual (as someone described above). If using more than one flash in ettl, a significant difference with the three-group mode (A:B + C) is that if the light from two or more flashes falls on a subject then the subject will be over-exposed. That is because in GR mode each flash operates independently of each other (so two or more on a subject will overexpose the subject). In the three-group mode the user can vary the ratio between two flashes but the system keeps the subject correctly exposed.

In manual mode, the new GR mode gives control over five flashes instead of three flashes on the previous system.

That's not accurate.

I use Gr mode with three groups all in ETTL at wedding receptions all the time. It permits me to adjust each group using its own Flash Exposure Compensation. Thus, I can have one off camera light as main, second off camera flash as kicker and on camera flash as fill. If one of the off camera lights is in the shot I can turn it off from the on camera control menu.

ETTL does not over expose a shot if more than one light strikes the subject. That's its benefit in this soft of situation. As I move closer to one of the off camera speedlites ETTL takes care to keep it from over exposing. Using the off camera lights in Manual would require me making constant aperture or ISO adjustments when moving around the lit area.

Here are two examples;
First is a wedding reception. Two 600s on stands back left and right, third 600 on stand front left. Exposure for ambient about -1EV maybe -1.6. All three lights in ETTL Gr mode.

Second was with 2 600s, on inside shower bounced into ceiling wall corner. Second was on stand bounced into wall camera right. Both in Gr mode, Ambient settings were high enough to eliminate it. FEC was set on the kicker to maybe +.3 EV.

The shot in the bathroom could have been done just as easily with manual speedlites or strobes. I used speedlites in ETTL just to be able to work faster. It was a boudoir session with three girls and I didn't want it to drag out. Reception shot would have been much more difficult with manual speedlites or strobes because I was moving from band member to band member with those on one side of the stage right under one light and those on the other side right under the other.


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apersson850
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Feb 14, 2014 10:15 |  #21

dmward wrote in post #16686267 (external link)
That's not accurate.

Yes it is.

At least the part that in group mode and E-TTL II, each and every group will by itself illuminate the subject to achieve what Canon refers to as standard exposure.

Thus if you set up a row of flashes, as many as you have, and give them groups A, B, C and so on respectively, you can easily test this. Select group mode and take a photo with the A group in E-TTL mode only. The other groups should all be off.
Then take the same photo with both A and B groups in E-TTL. Yet another with A, B and C in E-TTL. You can continue for as long as you have 600 EX-RT flashes, but obiously no longer than to group E.

Here are four images, all photos of a photo sitting on the side of a bookshelf. They are with one, two, three and four groups of flashes, each group consisting of one 600 EX-RT and each group set to E-TTL, when it's not disabled, of course.
No exposure compensation or any other changes between shots. It's easy to see that they get brighter and brighter as more flashes are added, even whithout seeing the histogram. When viewing that, it's even more obvious.

Group A.

IMAGE: https://lh6.googleusercontent.com/-IVbZ0uehMdE/Uv4_1yl4A3I/AAAAAAAATr4/_ZfNDVUCLwA/s400/AP1D8662.JPG

Groups A and B.

IMAGE: https://lh6.googleusercontent.com/-ze5c_ouebkQ/Uv4_2jIRR4I/AAAAAAAATsA/B7iXxdWT6uM/s400/AP1D8663.JPG

Groups A, B and C.

IMAGE: https://lh4.googleusercontent.com/-lH45xFvTaIs/Uv4_3L4JKxI/AAAAAAAATsI/zNOzZWBgdBE/s400/AP1D8664.JPG

Groups A, B, C and D.

IMAGE: https://lh4.googleusercontent.com/-fHkSESN7Gzg/Uv4_3vyHs_I/AAAAAAAATsM/ucg0_qIp2cU/s400/AP1D8665.JPG

As you wrote, you can get around this by applying various degrees of flash exposure compensation and/or by having various separation of what the different groups illuminate, but as a starting point, this is how it works. In my test case, all four where firing at the same point, from the same direction. In the samples in dmward's post, it seems likely that they illuminated different parts of the subject, like where one flash is to the right and the other to the left. Aiming all four the same does normally not make any sense, except to get more power, but I did it just to show the theory of operation.

Anders

  
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Roxie2401
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Feb 14, 2014 17:16 as a reply to  @ apersson850's post |  #22

Anders,

Question: I know its hard to tell, but I'm assuming that each of your units under ETTL would have fired a pre-flash, measured by the camera (the "through the lens" ability) and unless the camera is smarter than we think would have no idea how many flash units were being used, would have "told" all of them the same information - how much power, and it would have been the same - hence the over-exposure??

If the camera actually knew how many flash units there were and where the pre-flash came from - then it would have to have the intelligence to tell each of the units a different amount of power to use to keep the over-all exposure correct.........

Am I over-analyzing this?




  
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Feb 14, 2014 19:03 |  #23

Roxie2401 wrote in post #16689877 (external link)
Anders,

Question: I know its hard to tell, but I'm assuming that each of your units under ETTL would have fired a pre-flash, measured by the camera (the "through the lens" ability) and unless the camera is smarter than we think would have no idea how many flash units were being used, would have "told" all of them the same information - how much power, and it would have been the same - hence the over-exposure??

If the camera actually knew how many flash units there were and where the pre-flash came from - then it would have to have the intelligence to tell each of the units a different amount of power to use to keep the over-all exposure correct.........

Am I over-analyzing this?

This would require some sort of location detection, like GPS, being placed into each Slave. It would also possibly require an RMI transmitter (radio magnetic indicator) to be placed in the flash head so that you also knew the direction that each flash head was facing. This is probably the next step in wireless technology but Canon is not there yet. In the old optical wireless system the camera had no way of knowing how many flash units were in each Slave group but it seems to me, with the fact that only 15 total Slaves are allowed in Wireless RT and that those Slaves can communicate with the Master, that the Master and Camera could now know how many Slaves are in each group just not their location, direction the head is pointed or the type of modifier used.

The following warning is in the Canon 600EX manual on pg. 67 in the section about <Gr> mode. This is, as Anders showed, how the <Gr> mode handles ETTL calculations. I personally think it is not just a simpler way for Canon to handle ETTL exposure but a more creative way for the Photographer as well. In the wireless ETTL mode when using ratio A:B no matter how much you changed the ratio between Slave group A and B the overall exposure remained consistent as viewed in the histogram. In the <Gr> mode you are controlling the ratios between the different groups with the ability, from the Master, to control an individual groups flash output through FEC in ETTL or EXT.A modes or an actual manual flash output in Manual mode. Knowing that using ETTL in multiple groups will be an accumulative exposure allows the photographer to either compensate for that or use it to their advantage in a creative manner.


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dmward
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Feb 14, 2014 23:57 |  #24

apersson850 wrote in post #16689017 (external link)
Yes it is.

At least the part that in group mode and E-TTL II, each and every group will by itself illuminate the subject to achieve what Canon refers to as standard exposure.

Given that example you are right, just like if I setup 4 monolights, took a meter reading of each one individually, even though they are all next to one another pointed at the subject. Each light reads F11 so I set the camera to F11 and wonder why the shot is over exposed.

But then again, when would anyone ever do something that stupid?

Nice thing about Gr Mode is that if I need more light, I can put more speedlites in the same group and the camera will read the total output. If I want to be creative and use multiple lights from multiple angles to create modeling I can do that and the camera will read them individually and give me proper exposure. I discounted that there were photographers using the system with so rudimentary a skill set as to do what you illustrate.

I know you know better, but, as Rand points out, Canon realizes there may be some that don't. :-)


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Feb 15, 2014 05:57 as a reply to  @ dmward's post |  #25

Well, let's assume that someone is shooting in Gr mode, with several groups set to E-TTL. He then finds out that he wants to concentrate so much light to one specific place that he wants two units to work together. He thus places units grouped into B and C side by side, since they are both E-TTL anyway, and then get an overexposure, since the camera measures them individually.

Easy mistake to do, if you aren't aware that for this to work, you have to disable group C and switch the flash in group C, so it joins group B instead.


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Feb 15, 2014 06:00 |  #26

Roxie2401 wrote in post #16689877 (external link)
Anders,

Question: I know its hard to tell, but I'm assuming that each of your units under ETTL would have fired a pre-flash...

It's actually not difficult to tell. With four flashes in a row, the eye can very well see them firing their pre-flashes in sequence.


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Wavy ­ C
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Feb 15, 2014 21:27 |  #27

I think you are somewhat missing the point.

dmward wrote in post #16690503 (external link)
Given that example you are right, just like if I setup 4 monolights, took a meter reading of each one individually, even though they are all next to one another pointed at the subject. Each light reads F11 so I set the camera to F11 and wonder why the shot is over exposed.

We are not talking about monolights or flashes in manual mode, but multiple flashes in ettl mode.

But then again, when would anyone ever do something that stupid?

It is not "just like if I setup 4 monolights". With ettl speedlights multiple flashes can be set to point at a slngle subject and give the correct exposure (as in All or A:B ratio mode). The question is about how this works in GR mode.

Nice thing about Gr Mode is that if I need more light, I can put more speedlites in the same group and the camera will read the total output. If I want to be creative and use multiple lights from multiple angles to create modeling I can do that and the camera will read them individually and give me proper exposure. I discounted that there were photographers using the system with so rudimentary a skill set as to do what you illustrate.

I know you know better, but, as Rand points out, Canon realizes there may be some that don't. :-)

If each group in GR mode operates independently regarding exposure, I'm not sure if i follow your logic of using GR mode with three flashes in ettl mode (as you say you do at weddings all the time). Would it not be better to have the off camera flashes in manual and just the on-camera flash in ettl? Seems easier than having to constantly adjust exposure compensation on three flashes all the time?

Forgive me if I come across as one of those with the rudimentary skill set :)



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Feb 15, 2014 21:40 |  #28

apersson850 wrote in post #16689017 (external link)
Here are four images, all photos of a photo sitting on the side of a bookshelf. They are with one, two, three and four groups of flashes, each group consisting of one 600 EX-RT and each group set to E-TTL, when it's not disabled, of course.
No exposure compensation or any other changes between shots. It's easy to see that they get brighter and brighter as more flashes are added, even whithout seeing the histogram. When viewing that, it's even more obvious.

Interesting and thanks for posting this.

I wonder however if the flashes were in manual mode. So one flash in manual mode with the right power setting to correctly expose the subject. If three more flashes with the same setting were added, would the picture be more blown out?

Could ettl be limiting the amount of overexposure?

(sorry I don't have enough flashes to experiment with :) )



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Feb 15, 2014 21:54 |  #29

Wavy C wrote in post #16692511 (external link)
If each group in GR mode operates independently regarding exposure, I'm not sure if i follow your logic of using GR mode with three flashes in ettl mode (as you say you do at weddings all the time). Would it not be better to have the off camera flashes in manual and just the on-camera flash in ettl? Seems easier than having to constantly adjust exposure compensation on three flashes all the time?

Forgive me if I come across as one of those with the rudimentary skill set :)

Its all a matter of practice and experience.

The reason I use ETTL with three speedlites, one on camera and two on light stands is so I don't have to constantly have to use FEC to get exposure right.

The FEC is used to set the ratios I want.

Problem with Manual is that as the subject gets closer to one light or the other it either has to be adjusted for power or the camera exposure has to be adjusted.

With ETTL the camera exposure control does the work.

As for knowing whether to use two groups or one to gang speedlites. It seems that should be intuitive. One group for one exposure.


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Feb 16, 2014 04:43 |  #30

Wavy C wrote in post #16692541 (external link)
I wonder however if the flashes were in manual mode.

Are you serious? You quote my post, where I write that all groups were in E-TTL mode, then ask if they were in manual mode?

Shooting a wedding, or something, with several groups in E-TTL can of course work, provided the flashes are reduced with FEC and/or not illuminating the same part of the scene. No magic there.


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