psvensson said:
Seems like you want contradictory things. Flare-prone lenses are generally low contrast, and high-contrast lenses generally have good resistance to flare. Since Zeiss is famous for its coatings, I would bet the Planar is very high contrast (even though it's actually made by Cosina). An old Summicron is probably not high-contrast.
You might think that efficient coating goes hand-in-hand with low flare, in my experience this is true for generalized flare but not for individual flare spots. The reference to Erwin Puts was interesting, he comments:
"It [the 50 mm Zeiss Planar] shares with that lens [50 mm Summicron] the weak suppression of secondary reflections, due to the reflections at the edges of the rear mount."
A while back I was doing a long series of pictures of backlit water with strong sun and at various times tried a new 50 mm Summicron, a new Nikon 50 mm f1.8 AIS and a Pentax 43 mm f1.9. The Leica lens had noticeably higher contrast than the Nikon lens, both had a tendency to flare spots, the Pentax was far better, even though it has what I would call normal contrast, not ultra-high, which is why I kept it and sold the others. I still have it and also a Nikon 50 mm f1.4, which seems much better. Among my medium- and large-format lenses, the ones with the lowest flare seem to be 4-element single-coated examples, although generally flare is much less of a problem with these than with 35 mm lenses. What I am looking for now is a lens like the Pentax in Leica M mount!
Thanks to all for responses!