Ok, The F---- is when you are manually setting aperture using the ring.
Bottom line -just leave it on A and set the aperture via the camera control.
FYI only... see: http://www.ausphotography.net.au/for...ad.php?t=30767 re: manual settings of aperture.
Tamron user manuals here: http://www.tamron.com/about/manuals.asp
The 90mm specific manual... http://www.tamron.com/assets/pdfs/272E.pdf
I want that lens! tell us how it goes for you!