Flash compensation with 10D, 420EX & Omnibounce ???

[J.A.F. Doorhof]

Yesterday I found something out which I would like to throw into this arena of experts :D.

I recently got my 420EX flash and use it to much delight with a white Omnibounce.

When making pictures I found out during Xmas that all my shots were only 75% on the histogram, lowering the shuttertime or aperture kept them at 75%.

I think this has to do with the TTL and the flash being compensated for the available shuttertime and aperture.

When choosing to add 1-2 to the flash compensation setting I have perfect histograms.

Now the questions,.....
Is this due to the omnibounce, or me bouncing of the walls, or ceiling ?
Shouldn't the TTL system got this ?

Greetings,
Frank

[nucki]

Hi!

I found out, that if I'm bouncing of the wall, the omni-bounce should be removed! I got much better results without the omni-bounce when bouncing from the wall.

I think the bounce takes lot of the light and if you are bouncing from the wall, then maybe the flash is on its limit!

best regards
Peter

[dtrayers]

You may have seen this but if not read about Canon EOS and E-TTL here:

http://photonotes.org/articles/eos-flash/

Depending on the ceiling height and subject distance you may be at the limit of the flash, but I think it's more likely how the camera is metering. If you do a "focus, recompose, and shoot" method you really should be using Flash Exposure Lock (FEL).

Still, I do think the camera is biased towards the left side of the histogram. I don't mind that. I shoot in RAW 99% of the time and I've found that I can easily get 2 stops more exposure converting the image with C1Rebel or Photoshop CS.

[scottbergerphoto]

J.A.F. Doorhof wrote:
Yesterday I found something out which I would like to throw into this arena of experts :D.

I recently got my 420EX flash and use it to much delight with a white Omnibounce.

When making pictures I found out during Xmas that all my shots were only 75% on the histogram, lowering the shuttertime or aperture kept them at 75%.

I think this has to do with the TTL and the flash being compensated for the available shuttertime and aperture.

When choosing to add 1-2 to the flash compensation setting I have perfect histograms.

Now the questions,.....
Is this due to the omnibounce, or me bouncing of the walls, or ceiling ?
Shouldn't the TTL system got this ?

Greetings,
Frank
Hi Frank,
A few thoughts:
1. Shutter speed and aperture do not directly affect flash exposure. They mostly control the exposure of the ambient light. The aperture by virtue of controlling the distance the flash can travel does limit the flash in that respect. Flash Distance(max)= Guide Number/ f stop. The Guide Number of the 420EX is 42meters. With bounce flash, you have to measure the distance to the bounce surface and from there to the subject. The Omnibounce will also cost you about 1/2 the distance as without it (The Sto-Fen site says it reduces your distance by a factor of 2.5 or about 2 f stops). ETTL is far from perfect. I find that regardless of which flash I use 420EX, 550EX, Quantum Q Flash T2D, I have to do some form of Flash Exposure Compensation +/-. With bounce flash and no diffuser I usually have to go +1 FEC.
3. As you correctly did, use the histogram and not the LCD to judge your exposure. I like to bring the right side of the histogram close to but not touching the end of the scale.
4. Canon Digital Cameras use ETTL, not TTL. The difference being that ETTL uses a preflash before the shutter opens to measure flash exposure using the camera's exposure meter. TTL measures flash with the shutter open.
5. The Omnibounce is designed to be used at a 45 degree angle. They don't recommend shooting straight on till you are beyond 15 feet. That would require a minimum f stop of f/4.0 with the 420EX taking into account the loss of two f stops. ( 15ft=126ft/f stop; f=8, f8-(2 stops) =Camera set at f/4) . Take a look at the Lumiquest Soft Bounce. It gives nice diffuse light without needing a surface to bounce off of.
Scott

[J.A.F. Doorhof]

Thanks for all the answers, I will try bouncing without and with the omnibounce.

And do some reading :D.

Greetings,
Frank