I use a moisturizer that contains SPF 50 on a daily basis, and mix a bit of foundation in there. Should I wear sunscreen separately?
Not necessarily. If the moisturizer's spf formula blocks both uva & uvb, you shouldn't need more sunblock. Further, the foundation may add a bit of physical sun blocking (like the zinc oxide people put on their nose at the beach). If the moisturizer blocks on one type of ultraviolet, then supplementing to cover the other type may be prudent if you're going to have significant sun exposure. Wearing hat helps, too.