I find some helmets with tight bends in the visor can cause blurry vision particularly with polarised optical glasses. Since I am as short sighted as a Muppet this affects me.

It's not so noticeable standing stationary in the shop (under artificial light) but as soon as you move into natural sun light it is.

At a guess, I'd say it could be poor light transmission in the plastic or poly carbonate. Usually a problem in the cheapest of the cheap units.