In article <4d5sfl$ja1@news.asiaonline.net>, wwho@asiaonline.net says...

>I am wondering how TTL flash work. I find that many people like
>to add a sheet of white paper in front of their flash to take portrait.
>If we set the camera in auto-mode, will it be underexposeure? Or the
>flash will stop the light until enough brightness is received. >Moreover, I want to know what is the function of slow syc with the >flash.

TTL flash works by receiving information from a meter cell in the
camera aimed at the film. In TTL mode, when enough light has hit
the film for good exposure for the ASA set, the flash cuts off the
light. Same happens in "Auto", but the meter cell is on the flash,
and you need to set a particular aperture on the lens. The sheet
of white paper (or any other add-on), if it does not make the light
source size MUCH larger than the flash alone, does nothing except
reduce light level (possibly necessary with close work to bring the
flash within its operating range), and is compensated for in both
TTL and Auto modes (unless the sensor in Auto mode can read off the
add-on [avoid that]). The synch. speed is the highest speed for the camera at which the shutter is fully open for the brief flash
exposure. Any slower speed may be used (to add ambient light to
the exposure, set up fill ratios, etc.) with the flash.
Rear curtain synch. is useful when ambient light exposure is present
and there is motion (and it is VERY useful when shooting people
at slow shutter speeds with fill-flash - the surprise of the flash
occurs at the end of the exposure instead of the beginning of the exposure, which can cause motion and blinks during the rest of the exposure).
Hope This Helps