ETTL with Omnibouncer

[robertpollai]

Hi everybody,

I am using a 430EX with an Omnibouncer from Stofen. My problem is now that with normal use (i.e. simply press the shutter button) pictures are usually getting to dark while when pressing the * (FE Lock) button first (triggering a preflash) the picture is usually fine.

I guess this has something to do with the preflash that is fired by shutter button not having as much energy as the preflash from * and since the Omnibounce takes away light the shutter preflash is not enough to properly measure the required flash output.

However, what I want to ask if this is normal and everybody experiences or if it is something with my setup.

BTW, besides the aforementioned flash I am using a 30D with an EF-S 17-55/2.8.

Thanks and regards, Robert

[scottbergerphoto]

You are going to get a preflash whenever you are in ETTL whether you have an omnibounce or not. The power of the preflash as well as the max effective flash itself is reduced by the diffuser.
From Stofen's site:
We have found that with the automation offered in most of today's flashes makes exposure quick and easy. No exposure adjustment is needed on your part. The only thing that happens with the Omni in place is the Maximum Distance of the flash is reduced by a factor of 2.5. For example if your Max Distance is 50 feet divided by 2.5 now equals 20 feet with the Omni in place.
If you find that your pictures are underexposed using the Omnibounce, be sure you are using it properly, are within the max distance for your flash with an Omnibounce attached, and use FEC as needed.

[Curtis N]

It has nothing to do with the pre-flash.

It has everything to do with what the camera is metering.

Normally the camera measures light from the pre-flash throughout the entire frame in one way or another. You can choose between Evaluative and Average (custom function 14) and there are differences between them, but in either case, the flash illumination of the background will affect the metering.

When you use Flash Exposure Lock, the camera uses only the center of the frame (approximately the area of the spot metering circle) to meter the flash. The color/brightness of the part of your subject in this area will affect the metering significantly.

Normally the underexposure issues you're experiencing can be overcome with +FEC, but every situation is a bit different. Flash Exposure Lock can be useful if you have a relatively neutral subject to meter on. I don't use it much, since it takes more time and tends to confuse people with the extra flash.

[robertpollai]

Hi Scott,

Thanks for your reply. I understand that I always get the preflash. What I am thinking is that the preflash triggered when pressing * is stronger then the one triggered when pressing the shutter button (because with the shutter button one the actual flash needs to be fired directly afterwards). If that is the case then the * preflash might be able to measure correctly even with the loss of light due to the omnibounce, while the shutter button preflash might not be able to do so.

I think that all the other parameters you mention should be OK, since the pictures look OK when the * is used.

What do you think?

Thanks and regards, Robert

[robertpollai]

It has nothing to do with the pre-flash.

It has everything to do with what the camera is metering.


Hi Curtis,

Thanks for the tip, I did not know that the FE lock uses a different kind of metering. I'll try to experiment a bit more with the information you gave me. :)

Thanks and regards, Robert

[robertpollai]

Hi again,

I changed the custom option 14 to "Average" and now the pictures with and without FE Lock are more consistent, i.e. the ones without FE lock are much better then before.

Thanks for the advice (and for the flash photography 101 link, reading up on that now ;)).

Regards, Robert