Beginner Canon Speedlite question

[RomneyTansley]

I've been using Canon Speedlites for portraits of family and friends and learning in the process how to control and balance them using the built in wireless system. One thing I do is control an off camera flash (slave) with an on camera flash (master) which is disabled. The 'firing group' is therefore A:B. However the setup on the back of the camera still gives me the option of setting the A:B fire ratio. This seems silly considering that the A Speedlite is disabled! How can there be a ratio between two things when one is disabled? I realise there may be something about this that I'm not understanding so I'd be grateful for any enlightenment. Thanks.

 

[Canon Bob]

Whilst you've disabled the group A flash mounted on the camera, the camera has no idea whether you have any remote flashes also in group A.

Bob

 

[RomneyTansley]

Thanks, that hasn't occurred to me. Does this mean that as long as there are no other group A Speedlites, then altering the group A:B will make no difference? Thanks.

Romney

 

[Canon Bob]

TTL/ETTL will have multiple pre-flashes depending how many groups are configured. Group A will fire (and the exposure measured), followed by Group B (and the exposure measured) and so on until all configured groups are done. If the required ratio is 2:1 (A:B) then it will set the flash output accordingly.....ie, Group B flashes will produce half the illumination of group A (assuming that they were not combined).
This is not the output power of the flash as the distances from the flashes to subject aren't known.

It would appear (although I'm not certain) that if the Group A pre-flash doesn't change the illumination from the ambient metered value then the Group B ratio doesn't reference Group A..... a 2:1 (A:B) setting would mean that B is 1/2 of nothing.

Bob

 

[SeedyBee]

One thing I do is control an off camera flash (slave) with an on camera flash (master) which is disabled. The 'firing group' is therefore A:B. However the setup on the back of the camera still gives me the option of setting the A:B fire ratio. This seems silly considering that the A Speedlite is disabled!

Source: Some Canon patents.

The Master flash mounted on the camera is in an unlabelled fourth group. It is not "in" group A. It borrows the settings of group A when it is set to fire for the shot, as there is no human interface on the camera to make such settings for the Master.

Firing groups A and B on pre-RT cameras are always on; only group C can be disabled. The programming logic looks after a zero light contribution, like A=0:B=X produces B = sole non-ambient lighting (not half if output is sufficient).

Of course, an intelligent trigger can lie to the camera about its capabilities, and translate and extend them to do non-Canon tricks.