Automatic Wireless Flash - reprise

From reading many of the other threads/posts here, I thought I kinda understood this subject...until I went got one add'l flash and play with it (Iam/was planning to get a 3rd flash). On reading the 580 EX manual, I saw the following:

- The 'A' and 'B' flash groups can be set to ratios ( 1:1, 1:4, etc in 1/2 stop increments) of +/- 3 stops. This can be with or without the master (on camera) unit firing or not (580=off or ST-E2). This was as I understood it.

What I did not understand was the info on pages 40-41:

- "...You can use...slave group C to illuminate the background to eliminate shadows..."

- "...For slave ,C>, also set the flash exposure compensation as necessary..." In setting the 580EX, it appeared that the settings are +/- 3 stops in 1/3 stop increments (but not as ratios).

- "...if you point the slave group <C> speedlite toward the subject, the subject will be overexposed..."

Please help me understand.

As I read this, only the A & B groups will be a part of the subject exposure calculation. Group C does not appear to be in the calculation.

Also, what is the 'C' exposure comp (I guess FEC) relative to?

It almost seems that if you want the subject lighted stronger on one side that the other (by flash, not reflector) you are forced to have either 3 flashs (at least one master) or an ST-E2 and 2 flash units....is that correct. And 'C' is only for "non-subject" lighting.

Appreciate all your assistance/comments...​.I'd really like to understand this before I go off and buy the wrong stuff )though why should this be different )

John

The simplest wireless set up with Canon EX series flashes is using two flashes-one Master on camera(Defaults to A), and One Slave set to B, positioned off camera. You set the A:B ratio something like 1:2 or 1:3 so the on camera flash is the weaker fill light.
When you add a third group or C: (A:B):C
C is designed to light a background and not the subject. The ratio is (A:B):C. So you are deciding how much light to illuminate the background relative to how much light illuminates the subject. The camera is programmed to properly illuminate the subject with A+B and expects C to be used for something else. So if you point it at the subject, it will add to A+B. ETTL uses a preflash to determine flash output as opposed to TTL which measures flash in real time. A TTL system could avoid overexposure in this scenario, whereas ETTL cannot. You set C relative to the total amount of A+B. Setting C at +1, means you want the background 1 stop brighter then the subject. There are obvious limitations to this way of setting up lights.

I have moved to using wireless Manual. Press and hold the MODE button on each flash (set to Slave)for 2 seconds until M flashes. You can then set each flash output manually and use the Master to fire them. You can also do the same thing by setting Manual outputs for each flash on the Master. This procedure requires using a flash meter. If you have the time, this is a more accurate method.

Thanks, Scott

Am I correct that the 420EX cannot be manually set? Which means using either 580EX or 550EX. And returning, if possible, the 420 I just got from B&H...and added expense

John

PS...I am guessing, but can the 420 be set by the master, in the case where you said:

jrsforums wrote:
Thanks, Scott

Am I correct that the 420EX cannot be manually set? Which means using either 580EX or 550EX. And returning, if possible, the 420 I just got from B&H...and added expense

John

PS...I am guessing, but can the 420 be set by the master, in the case where you said:

Unfortunately, according to the 420EX Manual, Page 43, the Master units mode must be set to ETTL for the 420EX to fire as a slave. The 420EX itself, no Manual controls. So you can't use the 420EX in Manual or wireless manual.