2 speedlight power settings

I'll be using a 550EX on camera and triggering a 430EX off camera and I'm wondering if I use the on camera flash exp comp, does that apply to both flashes or only the one on the camera?

Is the off camera flash power always set manually even if I up the FEC on the camera?

The FEC, either set on the master 550 or in camera, will affect both flashes.

The off camera slave flash in ETTL mode is controlled 'automatically' by master unit and the camera flash sensors.

Remember FEC is to tell the camera what tone/reflectivity the subject is, not change the relative power of each flash. Use ratios for that.

Tim, are you saying that other uses of FEC are not valid?

tim wrote in post #6199244
Remember FEC is to tell the camera what tone/reflectivity the subject is, not change the relative power of each flash. Use ratios for that.

Wow, that's what I was thinking {change the relative power of each flash}. I guess I need to read up on this stuff again. It's been awhile.

jseivert wrote in post #6199973
Wow, that's what I was thinking {change the relative power of each flash}. I guess I need to read up on this stuff again. It's been awhile.

You can use the FEC on each slave flash to control its output but it's easier to do that by setting the ratio on the master instead of by setting the FEC on each flash.

CliveyBoy wrote in post #6199942
Tim, are you saying that other uses of FEC are not valid?

I'm not saying they're invalid, i'm saying people who use it other ways may not understand their equipment very well and could probably benefit from some reading. If you don't understand your equipment you'll be forever puzzled and will lack the confidence that understanding will bring.

Sometimes I do use FEC just to make the flash go brighter, but in general FEC is used to help the camera with its metering, not to manually increase power. It's a subtle difference, but once that people may benefit from understanding.

jseivert wrote in post #6199973
Wow, that's what I was thinking {change the relative power of each flash}. I guess I need to read up on this stuff again. It's been awhile.

Ratios ftw.

So if I understand this now: If I change the FEC on the camera to +1 I'm not increasing the output of the flash by 1 stop, I'm making the camera 1 stop more sensitive to the light that the flash is putting out. Kind of like upping the ISO or opening the aperture, while other things are constant.

jseivert wrote in post #6203201
So if I understand this now: If I change the FEC on the camera to +1 I'm not increasing the output of the flash by 1 stop, I'm making the camera 1 stop more sensitive to the light that the flash is putting out. Kind of like upping the ISO or opening the aperture, while other things are constant.

No. If you set FEC to +1, the flash will output 1 stop more light than the camera thinks the flash should. If you set the FEC on the camera or the master, then all flashes will increase the output by 1 stop. If you just set FEC on the slave, only that slave will increase its output by 1 stop.

tim wrote in post #6199244
Remember FEC is to tell the camera what tone/reflectivity the subject is, not change the relative power of each flash. Use ratios for that.

So this is not right?

PacAce wrote in post #6203983
No. If you set FEC to +1, the flash will output 1 stop more light than the camera thinks the flash should. If you set the FEC on the camera or the master, then all flashes will increase the output by 1 stop. If you just set FEC on the slave, only that slave will increase its output by 1 stop.

I'm confused again. Just when I thought I had it.

(and I guess we're going down a different road now than the original question)

FEC is used to increase/decrease the power of the flash units relative to what the camera thinks they should use. Ratios increase/decrease the power in relation to each flash.

Titus213 wrote in post #6205030
FEC is used to increase/decrease the power of the flash units relative to what the camera thinks they should use. Ratios increase/decrease the power in relation to each flash.

And to expound on what Dave posted, when using ratios, despite what the power level relationship is between the master and the slave, the overall exposure will still be what the camera determines to be the "correct" exposure. You will still need to use FEC to override that exposure if you want the exposure to be dimmer or brighter than what the camera wants it to be.