Strobist question

Question

When I am using my 580exII on camera and am using the built in IR to trigger two other 580exII off camera
AND
I am using ETTL on all units with the on camera as the master
one off camera speedlight set to A and one set to B
I can balance A B on the camera menu no problem

Ok so here is the question
if the metering is done by a preflash on the master unit
first why does the preflash not set off the slaves
second will the preflash set off a monolight (too soon)
third if the master takes a reading from the preflash determines how to distribute the flash to the slaves by the ratio set in the menu
Does it know how far my slaves are from the subject?(flash fall off is a square of the flash to subject distance)

Wow, Ind, these are some loaded questions---But let me share some of my experience---

1st--I have no experience with monolights, but My guess would be no, unless the monolight has a simple "slave mode" and in this case it fires off of the light emitted by the flash and not the IR. I don't know if there are any monolights that are compatable with Canon's IR system.

2nd, The slaves should fire from the "test shot" not neccessarily the pre flash. If they are not firing from a test shot, then you need to investigate other problems or set-up issues.

3rd, I do not know the physics/electronics behind the system, But I can tell you how it behaves when I have used multiple flashes with ratios established.

so......when using ratios, it is much like using flash compensation meaning that in ETTL mode, the system may give you an accurate exposure, but that exposure may not be what you are looking for artistically. So, you make adjustments based on your artistic desires not necessarily "accurate" exposures. For example: You may choose to have one flash "accurately" expose one side of your subject while just adding a little fill flash to the other side to reduce some of the shadowing. In this case you would establish a 1:2, 1:4, 1:8 ratio----depending on your desires.

Regarding the inverse square rule, I use it heavily when shooting in manual flash mode, and I don't even consider it when shooting in ETTL mode, especially if I am using ratios. But If I had to guess, I don't believe ETTL measures distance, instead, It measure light bounce off of the subject.

When I first started using multiple flashes, I hated ETTL and chose to stick with manual mode only. But after some time and experimenting, I realized that, like anything else, it has it's uses and that it is very good tool when used properly.

I currently use ETTL when shooting birds/wildlife with a single flash for fill purposes. I also use it with multiple flashes and ratios when shooting macro.

I use manual/multiple flashes when shooting fast subjects like hummingbirds or water drops.

I know this didn't answer all of your questions, but hopefully it was helpful. Maybe someone else with more experience/technical knowlwedge will chime in.

Good Luck

I don't shoot people portraits, but If I did, I think I would try ETTL first, and if that didn't work for me, go back to old faithful manual mode