User Activity

I have been unable to change the ISO in the CF menu to get it to go to 100 or lower The lowest it will go is ISO200 What am I not doing correctly? Thanks
Is it worth the higher price for a UV filter, do they make that much of a difference with a higher quality L lens? What is a fair price to pay on a Quality UV filter?  I see them for as high as $300 plus.
I am looking to upgrade from a used mkiii to a mkiv is it that much of a difference ?  and is it worth the upgrade? Thanks