# What's a retina display?

I recently started using an old 1080p TV as my monitor for my gaming PC. Since I sit close to it, the immersion is nice when playing Assassin's Creed. However the pixels are huge!

I was wondering if I would still see the pixels if I had a 4K TV of the same size. That led me to reading about visual angles, then about Apple's retina displays. If you want to read the Wikipedia articles yourself, here are the links:

Basically, for a given pixel density, usually measured in ppi, pixels per inch, as you get closer to your screen, there is a distance from which you will start to distinguish the pixels. So it is not enough to look at the pixel density, you have to also consider viewing distance to it.

One way to measure it is through the visual angle of one pixel. You measure the angle of a pixel of a certain size at a certain distance. Visual angle is also referred to as "perceived size" or "apparent size", depending on the field. I like "perceived size". And since it's an angle, it is measured in radians or degrees. They say that the eye can perceive down to 1 second of an angle, ie 1/60 of a degree. Below that, you won't distinguish individual pixels anymore.

On the other hand, Retina displays measure the number of pixels in one degree of an angle at a certain distance. Retina displays specify that you won't be able to distinguish individual pixels if for a certain distance, the pixel density is above 57 pixels per degree.

If you compare the two definitions, they are nearly identical. For a given screen at a given distance, if a pixel measures exactly 1/60 of a degree, that means there are 60 pixels in per degree.

Online calculators usually have clunky UIs, are full of ads and don't always compute everything that I am looking for. So I ended up making this table in Google Sheets to see in what settings I would or would not distinguish pixels. If it's less than 1 second of a degree, I won't:

Device Screen size (in) Distance to screen (m) Perceived pixel size (seconds of degree) Can distinguish pixels
iPhone XS 5.8 0.4 0.47 FALSE
Kindle (cheapest) 6 0.4 1.31 TRUE
Kindle (paperwhite) 6 0.4 0.73 FALSE
Kindle (Oasis, 2017) 7 0.4 0.73 FALSE
iPad Mini (2021) 8.3 0.4 0.67 FALSE
MacBook Air (Mid 2013) 13.3 0.53 1.29 TRUE
MacBook Pro 13 (2017) 13.3 0.53 0.73 FALSE
Dell XPS 15 15.6 0.53 1.14 TRUE
Dell XPS 15 (OLED 3.5K) 15.6 0.53 0.63 FALSE
Dell XPS 15 high-res 15.6 0.53 0.57 FALSE
Dell XPS 17 17 0.53 1.24 TRUE
Dell XPS 17 high-res 17 0.53 0.62 FALSE
iMac (2020) 24 0.63 0.65 FALSE
28 inch 1440p monitor 28 0.63 1.32 TRUE
28 inch 4K monitor 28 0.63 0.88 FALSE
32 inch 4K monitor 32 0.63 1.01 TRUE
42 inch 1080p TV 42 0.69 2.41 TRUE
42 inch 1080p TV 42 1.1 1.51 TRUE
48 inch 4K TV 48 0.69 1.38 TRUE
48 inch 4K TV 48 0.95 1 TRUE
55 inch 4K TV 55 2 0.55 FALSE
55 inch 4K TV 55 1.4 0.78 FALSE
77 inch 4K TV 77 3 0.51 FALSE
77 inch 4K TV 77 1.95 0.78 FALSE
1080p projector 136 3.5 1.54 TRUE
4K projector 136 3.5 0.77 FALSE

This has confirmed things I knew years ago, such as:

• I can easily distinguish pixels on the cheapest Kindle at 168 ppi, but the Kindle paperwhite or Oasis at 300 ppi is fine.
• I can easily distinguish pixels on older non-retina MacBook Air (Mid-2013 for example), and usually on other 13 inch laptops with 900 vertical pixels or fewer.
• I cannot distinguish pixels on 4K monitors, even the larger 32 inch ones. The table says otherwise, but it's borderline and it depends on the viewing distance, which is not perfectly precise.
• Phone screens have incredible pixel denstiy and I can't make out pixels even if I see them up close.

Some things I didn't know:

• 1440p, which is the standard for regular gaming PCs and XBox Series S, is not quite enough even for a 28 inch monitor. You'd see the pixels.

What's nice though is to see wildly different devices on the same table. Now I know that even with a 4K 48 inch TV, I would still distinguish pixels if I sit as close to it as a monitor.

By the way there's another notion called the viewing angle. It's the angle of the whole monitor to your eyes. I'll write about that in the next post.