Best NEX for MF?

1) I think all of the NEX series do. Happy to be corrected though. Certainly the 5 and 7 do. The recently released F3 and the 5R and 6 also support it. That only leaves the older C3.

2) 5 series works best for the widest range of legacy lenses because it's sensor doesn't cause as much colour shift on some of the RF WA lenses. However you can use any of them and then use Cornerfix to get rid of colour casts. No one knows how the 5R and 6 will perform yet because there aren't any reviews out on them.

I use a NEX 7 plus a range of RF lenses. The widest I have is a Zeiss ZM 25 2.8. I get magenta colour shift with that lens. My next widest is a CV 35 1.2 II and I don't see any significant colour shift in my photos with that lens, nor any of my others (50 and 85).

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Cheers
Ben