When the iPhone 6 and 6 Plus were first released, the PaintCode app folks put together this web page to demystify the various screen sizes and pixel configurations. Great stuff.
But Ole Begemann was not satisfied with that stock analysis. He dug deeper, using a closeup camera rig to take pictures of the iPhone 6 Plus screen doing its downsampling magic. Pretty pictures, and he published all his code on GitHub for folks who want to try this themselves.