Question about flash exposure

Hello there,

I have an XTi-400D, and I started to play with the built-in flash, before buying something more appropriate.

I still can't figure what is the exposure rule of the E-TTL II, even after reading tons of documents on the net.

That is, in natural light the exposure is calculated to match a 18% neutral grey, the mean value of the picture is around 118.

If I use the flash, what is the mean value that the camera tries to reach ?

If I fill the frame with a white wall, and take 2 pictures, one with the flash, one without the flash, it would seem logical to me to get the same exposure. But the flash picture is much brighter, and I can't figure why !!

Thank you for your help !!!

"mean value of 118" makes no sense. ETTL works similarly, but you have to remember you're balancing light sources.

With your test it seems like you're in P or Av. It ignores the flash for metering, so the flash adds extra light - in Av mode the flash is always fill, not the main light. So you're adding light, not taking any away, so things are brighter.

Read the EOS flash bible: http://photonotes.org/​articles/eos-flash/

ETTL is itself somewhat arcanely programmed, even the most experienced of photographers are baffled by its behavior at times, as the rules it follows are a bit like learning the rules of spelling in the English language...so many exceptions!

If you pointed to an 18% grey card only, the ETTL would emit a preflash and try to expose to 18% overall level. It is the average scene which throws in zingers which baffle ETTL. For example, if the preflash sees a highly reflective zone, it underexposes so as to not blow out the highly reflective zone, leading to underexposure. That is one of the reasons some choose to put the camera into Average Flash, not Evaluative Flash mode via the CFn.

Also there is a phenomenon known as NEVEC or Negative Evaluative Exposure Compensation (NEVEC) The phenomenon of NEVEC was discovered when users of the EOS 3 and EOS 1V reported that exposure settings in Av mode would change as soon as the flash was switched on. By careful measurement with a 1V, someone found that automatic ambient reduction was not a function of EV, but of light levels. And the amount of exposure change is ISO specific. Flash NEVEC appears to be programmed into the camera body, rather than being a feature of any specific flash unit. Here's a thread about NEVEC.

https://photography-on-the.net/forum/showthre​ad.php?t=107588

In addition to what Wilt said, and I hinted at earlier, the flash behaves differently depending on the metering mode you're in.

Thanks for your answers, guys.

It's still not so clear to me. I still don't understand why I get different light levels when I fill a picture with a grey sheet, with or without flash.

Sub35 wrote in post #5982907
Thanks for your answers, guys.

It's still not so clear to me. I still don't understand why I get different light levels when I fill a picture with a grey sheet, with or without flash.

Because your camera's built-in meter is dependent on reflected light. It doesn't know what color your subject is, so if it sees a lot of light coming back from a white subject, it'll underexpose, and vice versa.

Sub35 wrote in post #5982907
Thanks for your answers, guys.

It's still not so clear to me. I still don't understand why I get different light levels when I fill a picture with a grey sheet, with or without flash.

If you meter a gray card with a reflective ( your camera ) meter and the light on the gray card is the same as your subject, you will get an exposure that very closely approximates what your eye sees and what an incident ( ambient ) light meter will suggest as an exposure. In reality color doesn't mater, it's value. Thousands of different colors have the same value.

Cheers

Besides what Wilt said, sometimes things look different because of the quality of the light - color temperature, direction of the source, level or lack of diffusion and so on, even on a flat card or a sheet. But lordy yes, ETTL can seem to be plum goofy sometimes.