In article , graphic@delphi.com says...

>I found out the hard way (using slide film) that my Nikon N2020 + Vivitar
>436AF flash were intermittently not registering TTL exposures properly....
>out of 8 rolls of film, all were perfect....except even while shooting
>same setup suddenly got massive overexposures on certain frames while
>others perfect. Usually shoot negs so this went unnoticed previously.
>Started using Viv flash units audible underexposure alert tone and
>found (sure enough) it was going off even when disytance was well within >flash range and negs were getting overexposed.
[...]
When I used SB-15's and N2000's, the flash would sometimes move off
the TTL pins just enough to lose TTL control, and I found no way to
reliably prevent this (the F3 shoe locks the flash foot in place, so
this is never a problem). Since switching to a pair of 8008s and SB-24s,
I have not had the problem (the SB-24 foot is thicker, and fits more
tightly than earlier flashes).
Hope This Helps