It is a commonly-held misconception amongst the general public that the opacity or sheerness (how much of the wearer's skin shows or doesn't show through) of tights is dictated purely by the 'denier' of the fabric. Firstly, the denier rating is one of mass, not thickness (specifically, 1 denier = the mass in grams of 9,000 metres of the yarn; the decimal equivalent, decitex, being the mass of 10,000 metres).
Secondly, the opacity or otherwise of a pair of tights is dependent on the density of the loops of the fabric, and their spacing, as much as the denier of the yarn, and also depends on the size of the wearer; the same pair of tights on two people of different sizes (not both at the same time! :-)) will expose more of the wearer's skin the tighter they are stretched, so the opacity will be seen to be different. As an example, we stock some sheer (i.e. see-through) tights of 110 denier, and some opaques of as little as 40 denier, so it certainly doesn't necessarily follow that more denier = more opacity.
So now you know!