Been shooting around with the M mount 35mm f1.7 Ultron on my M262 the last couple of days. To be honest, I think the thing is superior to the 35mm f2 Summicron ASPH. The Summicron is more compact, but they weigh about the same and the Ultron to me is just the superior optic. It certainly works great against the light, something the Summicron has failed to do every time I've tried using it that way the past 12 months.
I wonder if there's a compromise somewhere in the optics to keep the Summicron so small that contributes to the ineffectiveness against the light?