Thank you for the links, they are very informative. I read through
them, but I still have some questions:
1) Since I have the XT and 580EX and only use the center focus
point. In cases where I need to focus and recompose, should I do
so and then FEL using the * button or just allow the E-TTL II
system to take care of the flash exposure for me?
E-TTL-II isn't sensitive to recomposing. So you can simply focus and recompose. No problem.
There IS a difference between FEL and the evaluative metering though. In normal metering, the whole scene is analyzed. With FEL, only the center 9% of the viewer is used for the flash reading.
It will depend on the scene, and user preference which system you use.
2) The E-TTL II description is very vague, is there a better
description of it somewhere? I don't understand how it knows which
area it will "light up" if it reads all the metering zones.
Unfortunately the different explenations don't always agree completely. Here's what Canon says:
quote:
Here is how it works with the EOS-1D Mark II’s 21-zone metering: the ambient light is measured when the shutter button is pressed. Next, a pre-flash is fired and the metering sensor takes readings at the central 17 metering zones. The ambient and pre-flash readings are compared. The metering areas having a small difference are selected as the flash exposure metering areas. Areas with large discrepancies between ambient and pre-flash readings are excluded or down-weighted because they are assumed to contain a highly reflective subject, or the subject is not in that part of the frame. This assumption is also validated by the distance information, and the algorithm avoids chronic underexposure problems in such situations.
These readings are weighted, averaged and compared with the ambient light reading and the main flash output is then set and stored in memory.
Thus, unlike older systems, E-TTL II weighs and averages the flash metering, capturing the subject as a “plane” and not as a “point”.
As a result, EOS SLRs can obtain consistent flash exposures even if the subject contains various colors and levels of reflection. The camera also allows the user to select an averaged metering pattern through custom function settings.
***************************
(From
http://consumer.usa.canon.com/app/pdf/d_eos/syschart012904.pdf
NB: It's a usefull size, but BIG! It's 25MB.)
In my opinion: When looking at how E-TTL-II works, you should forget all about E-TTL. I think E-TTL-II is VERY similar to normal TTL. It just selects a few area's within the area of view, that shouldn't be used for the metering. To do that, it assumes that the ambient and flash return should be similar.
There's two reasons why it would not be the same:
1. an object is highly reflective. That can be seen by comparing it to the surrounding, and to the ambient.
2. an object is much in front, or behind the focus/flash plane. Thus being over- resp. underexposed by the flash. That can be seen from comparing flash vs. ambient, in combination with subject distance. (Ambient exposure shows gray level, which in combination with distance predicts absolute flash return.)
3) My understanding is that the flash metering is through the lens,
so even if I bounce or use a diffuser, the flash should properly
meter as long as I'm within the flash power range. Is this correct?
Yes, that is correct.
