In article <4ggsam$ieg@mars.hyperk.com>, glh@srv.net says...

>I have a Vivitar flash adapter for my F3 that appears to support TTL >flash. At least it carries all the camera contacts through to the
>shoe, and my Vivitar 550FD appears to have its output controlled when >set to the "TTL/manual" setting. (I don't have any pictures from this >combination to prove that it works RIGHT.)

Ah, the infamous Vivitar F3-to-Nikon-ISO-TTL adapter has appeared
again! ;-) While all the F3 TTL contacts are connected through the
adapter, what is missing on ISO footed flashes is the mechanical
"feeler" that contacts the tab on the F3 shoe to transmit the ASA
info from the F3 to the flash (ISO footed flashes have an extra electrical contact to do it). Some Vivitar flashes, like the 5600(?), allowed setting the ASA on the flash itself, but most flashes don't,
so they will default to ASA 100 or 25 (which?) when used on the
Vivitar adapter, alas.
Hope This Helps