Tamron 28-75

The IF keeps the front element from rotating so that graduated type filters don't have to be adjusted once they are set. I use a circular polarizer frequently, nice feature.

I think the "SP" is the label for Tamron's new "Super Performance" line up.

EDIT: Another advantage of the barrel not moving back and forth during focus is for "balance", I don't see where that feature is a big deal to me, maybe others though.

You might do, if you want to do Macro in order to not scare your object, or if you are using a circular polarizer (many lenses also turn when they change the length)

Best regards,
Andy