What is a ‘Retina’ display?

Apple describes some of its products as featuring a “Retina” display. But what does that actually mean?

The individual pixels (each one made up of three red, green and blue subpixels) that make up my laptop’s display, viewed through a magnifier.

The main claim that Apple makes of its Retina display is that the pixels used are so small that they are too small to be seen individually by the human eye. In physics terms, this means that these pixels are below the resolving power of the human eye.

The resolving power of the human eye is about 60 arcseconds, or 0.0167 degrees. This means that any two objects separated by an angle smaller than this will appear as one object to the eye. The minimum vertical or horizontal spacing between two items which are visible as separate items is therefore given by dtan(θ) where d is the distance to the items and θ is the resolving power of the eye.

Assuming that the display in question is held or viewed at a distance of 30 cm from the eye, this distance is found to be 0.0873 millimetres. This means that a person with normal vision will be able to discern individual pixels on any display with fewer than 11.5 pixels per millimetre.

As can be seen from the graph above, the screen of the iPhone 4 does possess a greater density of pixels than the human eye can perceive; but the iPad 3 and the just-released 2012 MacBook Pro do not. (None of this matters of course, because “Retina” is just a trademark that Apple uses as a marketing term.)

An argument could be made, in the case of the MacBook Pro, that the distance between the screen and the eye would usually be larger than 30 cm. If the distance was 50 cm that would make the resolution of the eye 6.88 pixels per millimetre and therefore give the 2012 MacBook Pro a “true” retina display.

3 thoughts on “What is a ‘Retina’ display?”

1. Rowan Lewis says:

Interesting, although 30cm is rather close to the screen. I just measured my current distance to be roughly twice that.

So I guess it’s good enough, at least for me.

2. Re 30cm from screen of a macbook – I’m a good 60cm from mine at the moment and it’d be uncomfortable to be closer. I think it’s safely “retina”.

3. I don’t think I’ve ever witnessed anyone sit within 30cm (1 foot) of a screen that big (or even as big an iPad screen). I’ve just done a quick spot check around the office and oddly, the people I checked are all to sat 50-60cm from their screen. Some of that may be a factor of the length of the adult arm (for typing comfortably). So that distance is very seems very constant. People might lean in now and again when looking at a picture or something, but nobody actually sits 1 foot away from their screen for any length of time.

Also, if 226ppi resolution wasn’t sufficient to be “retina” then it’s also worth noting that would also imply that we should see the Red, Green and Blue subpixels as a stripey rainbow pattern in most desktop monitors, but I’ve never heard anyone complain they see the coloured subpixels separately. They just blur together to make white…