Flash ratio question when using bounce

Hello everyone,

I'm using a 30d with a 580 exII (used as fill) and a 430 ex as a slave (used on a lightstand as the keylight). I usually bounce the fill flash, pointing it straight up, and pulling out the little white plastic card. The slave I put at various angles around the room.

I started using a 1:4 ratio but many times the slave seems to overpower the fill too much, so I turned the ratio down to 1:1 and it looks better, even though the slave is still brighter than a 1:1 ratio should indicate. My questions are:

1. Is ETTL-II smart enough to maintain the correct flash ratios even when using bounce, or does it simply send the same amount of power to the fill flash whether I'm using bounce or not?

2. When adjusting FEC on the camera, does it affect both the fill and the slave?

3. Can I adjust the slave flash either on my camera or on the slave itself, without affecting the ratio?

John,
Wireless ETTL control only knows the RELATIVE OUTPUT power from flash units; without a photosensor of its own on each flash, it CANNOT know the EFFECTIVE output power that results from bounce against adjacent surfaces.

Wilt is correct. For a more thorough explanation, let's hope PacAce see this and elaborates.

John E wrote in post #4566227
Hello everyone,

I'm using a 30d with a 580 exII (used as fill) and a 430 ex as a slave (used on a lightstand as the keylight). I usually bounce the fill flash, pointing it straight up, and pulling out the little white plastic card. The slave I put at various angles around the room.

I started using a 1:4 ratio but many times the slave seems to overpower the fill too much, so I turned the ratio down to 1:1 and it looks better, even though the slave is still brighter than a 1:1 ratio should indicate. My questions are:

1. Is ETTL-II smart enough to maintain the correct flash ratios even when using bounce, or does it simply send the same amount of power to the fill flash whether I'm using bounce or not?

2. When adjusting FEC on the camera, does it affect both the fill and the slave?

3. Can I adjust the slave flash either on my camera or on the slave itself, without affecting the ratio?

I personally have never had problems with ratios between key and fill light whether I was using them directly (which is very seldom), shot into or through umbrellas or bounced off ceilings.

Can you give a little more specifics on what settings you used? I'm assuming the 430EX was set to Group B? I'm also assuming that the master was mounted directly on the camera. How far was the camera from the subject? How about the slave? And, most importantly, do you have a sample shot we can take a look at.

Here's something else to try. With the same settings you had (either 1:1 or 1:4), take one shot with the master turned off so that we can see what the slave lighting looks like. Then take another shot with the slave turned off so that we can see what the master lighting looks like.

John E wrote in post #4566227
1. Is ETTL-II smart enough to maintain the correct flash ratios even when using bounce, or does it simply send the same amount of power to the fill flash whether I'm using bounce or not?

ETTL determines the amount of light that's reflected back at the camera by the subject so, assuming that both the fill and the key lights are lighting the subject equally, ETTL will be able to set the lighting ratio pretty accurately. However, in real life, things are not always that simple so there will be a margin of error in the actual lighting ratio that you end up with.

Also, if the scenario is such that you are working at the extreme limits of one or both of the flashes, the ratio may be way off, too.

John E wrote in post #4566227
2. When adjusting FEC on the camera, does it affect both the fill and the slave?

When you adjust the FEC on the master or on the camera, you are affecting the exposure of the overall lighting, i.e. both slave and master. But if the FEC is set on the slave flash itself, you are only affecting that slave output.

John E wrote in post #4566227
3. Can I adjust the slave flash either on my camera or on the slave itself, without affecting the ratio?

No. If you adjust the FEC on the slave, you will be affecting the overall ratio, too.