Output Power in Bounce Flash

[heheapa]

I think most of us have been using bounce flash.

Anyone know:
1. how's the ETTL/ETTL2 works in bounce flash?

2. what's the actual power output if you tilt your flash head to 45 / 60 / 75 / 90 degrees? How's that compared to normal mode?

Thanks.

[msowsun]

In ETTL, there is no set amount of power in bounce flash. It works the same way as in straight ahead flash. The power is automatically adjusted to get the proper exposure. As the subject gets farther away (or darker), the flash power will automatically increase until it is at max power. After that you will get underexposed shots.

Bouncing the flash will cause more light to scatter, and the flash will run out of power sooner, but it still works the same way.

If I had to guess, I would say the flash uses maybe twice as much power when bouncing.

[Jim M]

What msowsun said. And it all depends on room size and how bright or dark the walls and ceilings are and how far away they are.

[heheapa]

If the flash run out of power sooner in bounce flash, shouldn't it means more output power than it does in normal mode?

The flash and the camera won't sense how large of the room, how high of the ceiling, ..etc. So, how does the flash output power relate to that?

Let me ask the question in another scenario. Let's say we take photo in a open space, without ceiling and wall, eg. grassland of nowhere, and you are using a 50mm F1 lens, a full frame camera, and 2m from your model. With the flash head point directly to your model, you get power X. Now, with the flash head tilt to 45 degree, will the flash output power now be X? or Z = Y * X? And how about with 60/75/90 degree?

I know your model will get underexplosure with bounce flash as nothing for bouncing :) However, how does that affect your flash output power?

Thanks :D

[msowsun]

The power of the flash is controlled by the amount of light that is seen on the sensor at the time of the pre-flash. If there is enough ambient light seen by the sensor, the flash will barely need any power at all. (in straight ahead or bounce. it makes no difference)

If the sensor sees darkness, the camera will tell the flash to increase power, right up to max if necessary, to achieve a properly exposed scene.

In your scenario, the flash would simply go to max power but the scene would still be underexposed as there would be no way for the sensor to see the light from the flash.

[heheapa]

I just wondering how does the ETTL handle the bounce flash situation and if it will add certain power, eg. 1/3 EV when tilt to 60 degree.

Another question :)
In normal mode, your focal length will be shown in your 580 EX II. eg, if you are using 24mm, your flash will adjust according to that. What happen if you tilt your flash head to 90 degree? Will the flash still treat it as 24mm? or 50mm or other value? I think your flash should now show you -- mm in the lcd now. But what''s that --? Thanks.

[msowsun]

ETTL will always work the same way:

It doesn't matter if the head is tilted.
It doesn't matter if there is a diffuser on the flash.
It doesn't matter if the flash is pointing at a reflector.

All that matters is how much light reaches the sensor.

When the flash head is tilted, the head will automatically zoom to 50mm. (in auto-zoom)

http://i2.photobucket.com/albums/y17/msowsun/photo%20stuff/BounceFlash2.jpg

[heheapa]

Thanks Mike. That really clears my doubt :D

[msowsun]

You're Welcome :)

[Titus213]

ETTL - Evaluative Thru The Lens. The flash fires an evaluative burst of light which is analyzed thru the lens of the camera. Based on that the camera tells the flash how much power it needs based on your settings - up to the max power of the flash.

Mike's got it all right- it's just that since the flash is determined at the camera it doesn't matter where the flash head is pointed. Amazing that it all happens so fast.