metering mode and ambient and its affect on ETTL calculations

[craiglee]

both of these are easy to test, but i'm at work:

will ETTL calculate differently if i use spot, center, etc?

if i set my shutter speed to 250 in a well lit room compared to 1/10 i'm pretty sure the 1/10 will show more ambient tinting and may be more exposed with the same composition and lighting when using ETTL.

[apersson850]

The E-TTL system uses evaluative or center weighted for flash metering. Which mode to use is something you set separately from ambient light metering.

If the camera is allowed to control the exposure of the ambient light (Av mode, for example), then it will expose the ambient a little less when you add the flash, to avoid overexposing the total result. But if you use M mode, there's nothing the camera can do about the exposure of the ambient light. You have to take care about that yourself.

[Wilt]

Evaluative ETTL vs. Averaging ETTL, I don't think Center Weighted ETTL exists.

If the camera is allowed to control the exposure of the ambient light (Av mode, for example), then it will expose the ambient a little less when you add the flash, to avoid overexposing the total result.
Sometimes, but that does not say that a change in Tv or Av mode does not result. "Flash Negative Evaluative Exposure Compensation" (NEVEC) has been described by some as a Canon fill-flash engineering design choice to reduce the background exposure so the flash on the foreground 'pops'.
"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, I found that automatic ambient reduction was not a function of EV, but of light levels. In other words, it is a combination of EV and ISO. ...Flash NEVEC appears to be programmed into the camera body, rather than being a feature of any specific flash unit. Even the type of flash did not matter. On the Elan 7E (which has a popup flash), flash NEVEC is seen as soon as ANY flash is switched on, whether the flash is a TTL-only popup flash, an A-TTL EZ series Speedlites, or an E-TTL EX series Speedlite....One day I was using centre-weighted metering on my Elan 7E, and was pleasantly surprised that flash NEVEC did not occur! Further testing showed that flash NEVEC returned, but only when evaluative (and not partial or centre-weighted) metering mode was selected. The camera operating mode (Av, Tv, or M) did not influence NEVEC. "
The 5D suffers from NEVEC in all metering modes

[apersson850]

I see. I haven't systematically tested this feature, or if it is a pain, with all metering modes combined. But I've seen it happen on the digital cameras I have.