Retina is an interesting thing. At about 220 ppi it's different from most displays I've used, and is probably as good way to assess image sharpening on screen as one can get. 240-300ppi would be ideal, but it's not that hard to soft proof on a 110ppi screen either. 4K is going to be between 136-163ppi depending on 32 or 27" screen. Without spending Eizo or NEC Spectraview $$$, would be interesting to see what can be had that calibrates well down to about 120 nits, offers wide gamut (DCI-P3 or AdobeRGB) and offers true 10 bit color...
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