Okay I'll show my complete ignorance about the Zeiss lineup.
What is the difference between Planar and Distagon? I have seen these names often but don't know if there is significance to the names
I did a search and found:
Designed by Dr. Paul Rudolph in 1896 based on the double Gauss design (in 1817, C F Gauss described a telescope objective consisting of a pair of meniscus shaped elements, one positive, and one negative.) The design was 4 groups of 6 elements, and a flat field design. Symmetrical optical configuration producing low spherical aberration and astigmatism. The normal wide airspace separating the positive and negative elements in the double gauss design made a large amount of spherical aberration. Rudolph thickened the negative elements and reduced the airspace as much as possible, which corrected the spherical aberration and the sagittal/ tangential astigmatic aberration. Rudolph also inserted a "buried surface" into the thick negative elements of a cemented interface separating two type of glass having the same refractive index, but different dispersive powers. Not widely used until coating processes were available, due to light loss from the large number of transmission surfaces causing very low contrast. Due to it's complexity and high number of transmission surfaces, it really did not come into it's own until coating was developed. The planar was used as a base for lens derivatives, though in asymmetric form. Almost all the high-aperture lenses supplied on Japanese cameras are modification on the Planar..
This is a reversed telephoto lens, consisting of a large negative lens in front of an ordinary lens. This allows it to obtain a short overall focal length with elements of a larger and more manageable size, helps design a system that is favorable for both high relative aperture and wild-angular field, and increased the back focal distance beyond it's usual magnitude, which give space for the mirror of a SLR. The downsides are that is must be physically large, and of complex construction to correct all the aberrations, making the lens more expensive to produce. Reversed telephoto designs are rarely over 2 inches in focal length, and then it is only used for specific applications. Compared to the Biogon, it has a larger circle of illumination full aperture, though softer when wide open, though it is sharper when stopped down. Rear element does not interfere with mirrors in SLR's