You don`t need to go mad spending a load of dosh for a filter,but it would be daft not to have one,even to prevent a scratch to the lens-some folk suggest just using the lens hood,but that`s not always practical.
I got a 95mm Polaroid MC UV filter for my Sigma 150-600mm on Ebay for around £25-it`s really just a lens protector,and it seems to works fine.
It would be interesting to see what difference there is with the more expensive filters,price points start at £15-£30 then they jump to £100+
Probably like most things you get what you pay for.
