Does wearing sunscreen stop you from tanning? or just burning?

I would like to know if wearing suncreen stops you from tanning. At the moment, i don't wear any and can go for about 45 mins in the New Zealand sun without burning, and i get a tan relatively fast (im fairly pale). But is it better if i wear suncreen (i.e spf 20?), or does it lengthen the time i need to be in the sun (to achieve the same tan), what would be the best SPF (for someone fairly pale, but with a brown mother).