430 external flash/camera modes

I currently use the 350d with a new 430 ex flash..

I have read a couple of various articles/links from this site which have confused me. Can anyone answer the following queries in simple terms (please don't link me to a website - I can assure you I have read them and am no further forward)

I understand that in the auto modes (green square and p) the camera meters for the flash and will expose the foreground with the flash (background dark). Should these modes produce fill flash (which will may result in an underexposure of the foreground/subjects) or should it be a full flash (which will should result in a proper full exposure). My experimnts have resulted exclusively towards the latter (underexposure or the foreground/main subject) Why is this?.

I understand that in the manual modes (TV and AV) the camera meters for the ambinet light only (and sets apprirate shutter and aperture as if the flash wasn't been used). The flash then fires to hgihlight the foreground. What I am unsure of is does the flash fire a fill in flash only or a full flash?

In effect, in a dimly lit room this is a waste of time unless you have a tripod as the shutte speeds are too slow to prevent camera shake (unless a high iso is used (which results in noise).

I understand that manual mode is just that but again what will the flash do - a fill in or full?

craig2610 said:

I currently use the 350d with a new 430 ex flash..

I have read a couple of various articles/links from this site which
have confused me. Can anyone answer the following queries in simple
terms (please don't link me to a website - I can assure you I have
read them and am no further forward)

I understand that in the auto modes (green square and p) the camera
meters for the flash and will expose the foreground with the flash
(background dark). Should these modes produce fill flash (which
will may result in an underexposure of the foreground/subjects) or
should it be a full flash (which will should result in a proper
full exposure). My experimnts have resulted exclusively towards the
latter (underexposure or the foreground/main subject) Why is this?.

Click to expand...

Depends on ambient light levels. In typical dim indoors the flash will be the main subject light. In brighter outdoors the camera will automatically adjust the flash to be fill light. Under exposure of the subject when using flash in green box and P may be due to:
(a) Subject is beyond the usable range of the flash

(b) Using Focus/Recompose/Shoot without FEL (Flash Exposure Lock). E-TTL biases exposure around the active focus sensor. If you do not use FEL, then when you recompose the active focus sensor is no longer on your subject.

(c) Very reflective surfaces in the scene, or spot lights shining back toward the camera can cause the E-TTL metering to back off the flash exposure.

craig2610 said:

I understand that in the manual modes (TV and AV) the camera meters
for the ambinet light only (and sets apprirate shutter and aperture
as if the flash wasn't been used). The flash then fires to
hgihlight the foreground. What I am unsure of is does the flash
fire a fill in flash only or a full flash?

Click to expand...

Yes. ;-) Again, depending on the ambient light levels the flash can be the main light or fill.

craig2610 said:

In effect, in a dimly lit room this is a waste of time unless you
have a tripod as the shutte speeds are too slow to prevent camera
shake (unless a high iso is used (which results in noise).

Click to expand...

Yes for Av - though you can set C.FN 03 to "1", which willl peg the shutter speed to 1/200. Not for Tv, where you set the shutter speed. Tv however will tend to set the lens aperture wide open. Use M instead if you want to control both aperture and shutter speed. Use P if you want to maintain a hand-holdable shutter speed.

craig2610 said:

I understand that manual mode is just that but again what will the
flash do - a fill in or full?

Click to expand...

Yes, depending on ambient light levels and your choice of aperture and shutter speed, it'll automatically adjust between main light and fill.