clipped from: articles.mercola.com   
Shocking Update -- Sunshine Can Actually Decrease Your Vitamin D Levels
key messages

but the key take-home points here are: 

to optimize your vitamin d levels, get safe outdoor sun exposure or use a safe tanning bed, being careful not to get sunburned. 
if for whatever reason you’re unable to do that, then clearly you’re going to want to use an oral or sublingual vitamin d3, which can also eliminate the whole issue of absorbing the vitamin d3 from your skin. however, beware that anytime you use oral vitamin d you’ll want to get your vitamin d levels checked regularly, using a qualified lab such as labcorp, to make sure you’re within the optimal range and to avoid overdosing.
  • if you’re going to shower after spending time outdoors in the sun, don’t use soap for at least two days, to maximize the absorption of vitamin d in your skin.