In article <19970118203001.PAA18957@ladder01.news.aol.com>, charlied48@aol.com says...

>Flash units with sensors (like the Vivitar 283) are not TTL, and they
>measure the time/distance its light has traveled, bounced back off
^^^^^^^^^^^^^
>subject, and returned to sensor. Color or reflectivity of subject do not
>matter - ONLY distance. With TTL systems, the sensor is in the camera
^^^^^^^^^^^^^
>body and measures the AMOUNT of light reflected from the subject, not the
>time/distance. These TTL systems, while great for MOST subjects are not
>as accurate with abnormal reflectance subjects (like brides in white or
>grooms in black; or rooms with light walls and carpet). With subjects
>like these you will get over/underexposure, even in manual exposure mode.
>The flash (unless is has manual settings) is always under the control of
>the cameras TTL sensor and will output an amount of light that the sensor
>thinks appropriate for the f/stop set, no matter what reflectance the
>subject has. [rest deleted....]

Hmmm, this is, ah, er, a truly, um, unique, and, ah, *interesting* concept...;-) (But, why, to accomodate different aperture selection
for auto mode, are various strength filters or various size holes
dropped in front of the flash auto sensor, if light level is irrelevant
[just distance, um, *timing*...;-]?) Let's see, for a 3' timing (6' total flashtube-subject-flashsensor distance, at a light speed of about 186,000 *miles* per second) means that the timer in the flash unit would need to
be, uh, *VERY* precise, especially if we want it to accurately
differentiate between 6' and, say, 5.5' or 6.5'...;-)
(Sorry, just couldn't resist, but it's a neat concept anyway...;-)
Hope This Helps