> I find that some lenses have a different look on-axis versus off-axis, either leading to ugly smearing, or pretty twirly bokeh.
This is indeed pretty interesting. It seems like double-Gauss lens designs like the Zeiss Planar and Biotar are especially prone to this kind of distortion. There's a Russian copy of the 58mm Biotar called the Helios 44 that's popular on eBay for this reason (lots of people use an adapter to put it on a MFT camera and shoot video with it).
This is indeed pretty interesting. It seems like double-Gauss lens designs like the Zeiss Planar and Biotar are especially prone to this kind of distortion. There's a Russian copy of the 58mm Biotar called the Helios 44 that's popular on eBay for this reason (lots of people use an adapter to put it on a MFT camera and shoot video with it).