Actually the Planar (a trade name) is a dG. The dG can be traced back to a telescope objective designed by K.F. Gauss. The lanthanum glass Tessars can be very very good - another example is the f:3.5 version on the Rollei Ts, I had one that was really superb. But f:2.8 is asking a good deal of the Tessar design, regardless of the glass used. I prefer the F;6.3 Tessars for large format (such as the Commercial Ektars).
Originally Posted by Ken.Cartouche
The only Zeiss lenses (AFAIK) made in LTM were made during the war, they made the 50mm Sonnar(s) in a very small series.