Thanks, Ray, for your reference to the Takahashi. I was always curious about these, never tried one. According to hearsay, these aren't flat for photographic purposes, even though they are called flat-field. But this might refer to previous generations and I'd happily stand corrected. Are you US based? Where do you get these and the adapters?
As I said, Pentax has some good options in this range, and the 600mm/5.6 is quite nice. I do realize that there is still a tiny margin left for improvement
For all practical purposes, the 300mm Schneider APO is a wonderful lens and the 1.4x converter does not appear to degrade image quality at all. In fact, I keep the converter on the 300mm as part of my standard walk-around kit. I use this with the Sinar eMotion 75LV, which might explain why I don't notice any fall-off at all. Very sharp. Thanks, Geoff.
Talking about 250mm. Above 200mm, non-APO medium format telephoto designs all show some level of chromatic aberrations, which are only amplified in the digital world. Carl Zeiss recognized this early when they introduced the Superachromat. Rollei must have briefly toyed with adopting the Superachromat as there was a single reference in an old SL66 catalogue, but for reasons I cannot comprehend they never did. BTW – the 250mm Superachromat has the same level of light fall-off as the 300mm Schneider APO Tele-Xenar.
I did once compare all the 250mm medium format lenses in my arsenal and the winner was, surprise, the 250mm Mamiya APO 4.5 RZ, which I had hacked onto my Pentax 645D. It easily surpassed a rare Carl Zeiss-built (instead of the Rollei-built) 250mm Sonnar, which I have for the Rollei system.
I have never used the 180mm Tele-Xenar with the 1.4x converter, even though I have both. I have also not tested this or the 110mm Planar for resolution as it makes little sense. These lenses are certainly not designed for shooting brick walls. Others have compared the 180mm Tele-Xenar to the more recent 180mm f/4 Sonnar and found the Sonnar sharper, which seems trustworthy to me. The 180mm Sonnar is in another league than the 250mm Sonnar and among Hasselblad’s best lenses. The old Hasselblad… I’d still prefer the Tele-Xenar any day because of its speed and un-matched bokeh.
Those Schneider lenses for the Rolleiflex system commissioned by Heinrich Mandermann during his Rollei years are until this day the crème de la crème of medium format optics. The compact 40mm Super-Angulon, the fast 50mm/2.8, the 55mm shift lens. The 60mm/3.5 Curtagon, the 80mm Xenotar, and the 150mm Tele-Xenar all three are top in terms of sharpness and contrast. The 90mm APO-Symmar is perhaps the best medium format lens ever, closely followed by the 150mm/4.6 APO for macro purposes. Then the 180mm Tele-Xenar for portraits and the 300mm APO. Medium format lens heaven.