yup, i agree with bro d88p, the lens will turn darker with the amount of sunlight
and UV. ive been wearing them for abt 5 yrs liao .... in spore wont turn
as dark as sunglasses but when i was on the UK, wah.... very dark..
there are a few brands in the market, the one advertise on TV is on the high side.
try to check out the other brands when u visit the optic shops.