What resolution would a “Retina display” 27-inch screen be?
Retina 27” Thunderbolt display: 5120×2880 = 14,745,600 px
4K film: 4096×2160 = 8,847,360 px
Retina iPad 3: 2048×1536 = 3,145,728 px
This is based on the widely-held but fallacious belief that a “Retina display” is twice the resolution of a current-day display. There’s another widely-held but equally incorrect belief that to qualify as “retina” a screen must have more than 300 pixels-per-inch. I have previously debunked both of these for TUAW. In fact, Apple defines the term like so: “[a] Retina display’s pixel density is so high, your eye is unable to distinguish individual pixels”. This means the truth about what is and isn’t a Retina Display is more subtle, and demands consideration of viewing distance as well as screen size.
Let’s use the methodology I outlined in my TUAW post to consider the 27” Thunderbolt Display.
A Thunderbolt Display has 2560x1440 pixels over a 27” screen — that works out to 109 pixels-per-inch. If I had 20/20 (i.e. average) vision, I’d have to be further than 32” back from the screen to longer be able to resolve individual pixels.
I’m typing this on my 27” iMac right now and I’ve just measured the distance from the screen to my eyes as about 28”. This is probably a typical sitting position, At this distance, the finest detail I am able to discern is a mere 123 pixels-per-inch. In other words, my 27” iMac only has to go up to 2896x1629 (or some slightly higher and more palatable numbers) resolution to be a “retina” display at my current sitting distance. This is far lower than the resolution of a 4K film.
Footnote: I’ve glossed over some interesting details with regards to different types of visual acuity for different types of image, which you might like to read about.