I don't think the 150 2.8 BR is a new design. The basic lens design matches the Mamiya Sekor 2.8/150 IF which was introduced around 2008. They had to make the BR much bigger in order to fit the leaf shutter. The larger lens body would allow for better internal flare control which can be a substantial factor in wide open performance. They could have used more advanced coatings on the BR version and tightened the production tolerances. I haven't got any inside information on the actual differences. I'm just pointing out some options they could have employed to improve the lens over its predecessor without changing the basic lens design.
I posted the datasheets in this thread:
I would want to know the difference of the image quality in reality between the same design lens in different version of the Phaseone lens. e.g the 240mm Blue ring and the older silver ring lens with same optical design. Do you see any difference in real world usage? Any comments?
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