Biotar vs. Planar is used quite confusingly (to me). Have a look here:
http://photo.net/classic-cameras-forum/00CA9L
Both terms have their origin in pre-WW2 lenses based on double-Gauss 5 or 6 element designs (in 4 groups).
In later double Gauss designs, often, an additional 7th element was introduced for faster lenses to correct for abberations.
The Ernostar design was originally used by Ludwig Jakob Bertele working for H. Ernemann. It has four components of five elements, i.e., a first positive lens, a second cemented meniscus doublet lens, a third negative meniscus lens and a fourth positive lens. Later it was improved in the Sonnar design, that originally had 6 elements in three groups (two triplets, some-times replacing the center of the rear triplet with an "air element"). In contrast to Biotar both Sonnar and Ernostar ar asymmetric.
Today, an assymetric design with cemented doublet in the front will be called Ernostar more generically. And an assymetric design with cemented triplet in the front, will often be called Sonnar.
Also, the terms are non-technically used for marketing purpose, having nothing to do with design. The new ZM 85/2 "Sonnar" is a double Gauss design, for instance.
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Optically, the double Gauss design is typically used to get more even performance across the field. But many variations exist, optimized for resolution, bokeh, less distortion, etc.
Sonnars and Ernostars "feel" similar, in that wide open they have higher performance in picture center than at the borders. On the down-side, the focus plane is often curved, there is focus shift, etc. On the upside, bokeh is usually nice, they are very compact, and have little flare due to few glass/air transitions, only.
Cheers,
Roland.
