Of all the skin-care products we use, sunscreen is the most powerful — and perhaps the most scrutinized. To pass the basic buy test, it must be broad spectrum, SPF 30 or higher, and absorb without feeling greasy. One must decide between physical and mineral options and think about water resistance, too. And those considerations are just the start — after all, sunscreen is one of the only shields we have against premature aging from sun damage and skin cancer. But there’s one thing that even the most discerning of beauty nerds don’t know about SPF: which formulas the top dermatologists use on their own complexions.