A sunscreen with spf 15 have to be applied every hour. The sun makes it decipitate (wear down), when you are exposed to the sun. A sunscreen with an spf of 30 or 60 needs to be applied every 2 hours. Being a higher level means that it wears down less quick that the spf 15. And the spf 30's or higher are total block creams. So they offer a better protection. another thing when buying sunscreen> Stick to good brands: la roche posay, biotherm loreal, vichy dont' buy cheap ones like hawaiann tropic or banana boat. when it comes to your skin and protection buy the better ones.
yes its true that vitamin d cannot be absorbed through sunscreen. but if your drinnking milk or taking supplements your fine and eating your proper fruits and veggies.. Don't depend fully on the sun for vitamin D you'll be sorry.