In[]:=
SetDirectory[NotebookDirectory[]];
In[]:=
cyl=Cylinder[{{0,0,0},{0,0,1}},.5];
In[]:=
lens=ImplicitRegion[​​{0<=z<=2-.5x^2-.5y^2,x^2+y^2<=1},​​{x,y,z}];
In[]:=
Export["cyl.stl",cyl]
(*"cyl.stl"*)
In[]:=
Export["lens.stp",lens]
Export
:ImplicitRegion cannot be exported to the STEP format.
(*$Failed*)