--sunscreen--at least, effective sunscreen, blocks precisely those wavelengths of ultraviolet radiation that cause the skin's cholesterol to start the reaction that lead to the production of cholecalciferol (Vitamin D3).

From what I've read of the literature, though--mostly thanks to Mrs. D and Ted Hutchinson at the Vitamin forum at BT--by the time skin begins to redden in the sun, that means the skin can no longer add to its D3 production capacity--the reaction now breaks down into a homeostatic feedback equilibrium (e.g., whatever is made gets pulled apart almost immediately). The trick seems to be to get short, concentrated bursts of ultraviolet onto the skin and stop before there is noticable damage, whichis when the skin cancer incidence rises. Depending on one's own particular hue, the time of day, the time of year, and the latitutde, this may be anywhere from 10 minutes to a hour or more--in fact, at high latitudes, it's unlikely that during late fall through early spring one CAN get enough exposure to produce optimal levels (and there's a lot of evidence that in places like Scandinavia, people are running around with chronic Vitamin D deficiency, especially in winter).

Since I'm very light and freckly, in summer I limit myself to ten minutes--at high noon, maybe five. And this is with much of my body uncovered--short sleeves, shorts. In other months, there's less sun, and I am more covered, so I try to stay out longer, but I do supplement with D3, as I suspect I'm not getting what I need at those times of year.