Multi flash use with EOS R7 and 600 ex-RT flashes

[Rkeroack]

I have tried several times to use my R7 with two or three 600EX-RT flashes.  One flash on camera works fine.  A second or third as slaves always results in an under-exposed image, as if the on-camera flash is cutting power if it "sees" the other flash.  Is it that I need the hot shoe adaptor for the camera, or something else? Thanks!

[p4pictures]

Hi and welcome to the forums. Let me try to help you with this.

When you make a Speedlite a sender (formerly master) unit it becomes part of group A, receiver units (formerly slave) can be assigned to any of five groups A,B,C,D,E with Group mode, and three groups with other modes.

What this means is that of the on-camera Speedlite is set to illuminate the subject, and one of the off-camera units that is connected is in the A group, they will work together. If the off-camera unit is aimed back towards the lens and visible in the picture the camera will reduce the power output of the whole of group A to avoid underexposure of group A when using E-TTL mode for group A or all groups. It is also possible to configure the on-camera Speedlite to simply act as a radio trigger for the off-camera units and it won't illuminate the subject, but simply control the off-camera units.

I know that if a Speedlite is in the frame, or aimed back towards the camera then it is likely to be a problem, so I tend to put it in a group by itself and control that group in manual with the on-camera unit set to group mode.

To help me further answer your question, can you tell me what group you set the the off-camera 600EX to, and also tell me what mode you have on the 600EX on your camera. I need to know if E-TTL, M, Grp. Also if Grp mode, what mode you have set for each of the five groups. 

Also here's an example to consider...

For this photo I used three 600EX-RT units and an ST-E3-RT transmitter on the camera. Each 600EX-RT was set to its own group. The ST-E3-RT was set to Grp mode and controlled each group, and hence each Speedlite, individually.

• Group A - E-TTL - main light in a soft box on the models face
• Group B - Manual - kicker / hair light from behind the model to create the highlight on her right shoulder
• Group C - Ext. A - aimed at the background to lift the brightness a little 

[Rkeroack]

Thanks for the info, I do not have the second flash here at the moment.  I’ll have to check this.  The only odd thing is in the past with my 7D2, I could have multi flashes set up all on the same group and they worked just fine. The on camera flash was the main and the ones off to the side filled in and added some depth and shading as expected.  Any other ideas?? But I will look into your set up.  Unfortunately I am usually doing this on the fly and do not have too much time to dive into the menus.  I’ll have to see if I can get them set prior to the event and make small adjustments.

[p4pictures · Accepted solution]

Ah that explains some of your problems. EOS 7D and EOS 7D Mark II are unique in the Canon EOS range in that when the built-in flash is used to control Speedlites using optical wireless, the built-in flash is not treated as part of group A, in fact its a unique group in it's own right. This empowers the EOS 7D / 7D Mark II with the ability to control the off-camera lights independently of the built-in flash.

Also if you simply use the built-in flash on the EOS 7D/7D Mark II to trigger the off-camera Speedlites then it is possible to configure it not to be part of the exposure. With your EOS R7 and a Speedlite on the hotshoe you need to stop the hotshoe Speedlite from firing during the exposure. Locate the setting for sender flash firing and turn it off.

Alternatively set the sender unit to Grp mode, set the A group to E-TTL - this is for the on-camera light, and then set the B group also to E-TTL and make all your remote units receivers (slave in old terms) in group B. This will give you what you had with the EOS 7D Mark II.

[Rkeroack]

OK, that explains alot!  I will try that out at home before the next event.  Thanks for your help!