iOS lays out in points; assets ship in pixels. Convert either way at 1x, 2x, or 3x — with the iPhone Duo's exact display sizes as presets.
1398 px
466 pt
Both Duo displays render at @3x, but their point sizes differ wildly (466 × 678 vs 669 × 951). Exporting a @3x asset for the wrong display means a blurry or oversized image. Full numbers: device specs.