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!