Screen size

XKCD made an observation last week:



I find it impressive because I can sit on my couch and watch well detailed images, not upright in my dining room chair at the laptop, or staring down at the iphone on the table or in my lap. Apparent pixel size makes a big difference here.

Let's take a look at the 24" Apple Cinema display. It has a 1920x1200 native resolution. If you put that at the arms-length recommended by ergonomicists, it's 28" away for me. What does that mean? It means an apparent horizontal pixel width of 0.022274 degrees, that's what. And the math:

Actual horizontal width per pixel = 1920 / 20.9" (the actual width of the screen) = 0.010885"
Distance to that pixel = 28"
Angle = tan-1 ( 0.010885 / 28 ) = 0.022274 degrees

Lets say I have a 42" HD-TV at home that sits 8 feet from the couch. That's a 1920 horizontal resolution at 96 inches. Giving an apparent horizontal pixel width of 0.011376 degrees, markedly smaller than the Apple display at 28".

An iPhone at 12 inches has an apparent pixel size of 0.034817 degrees. Just so you know.

Generally speaking, smaller apparent pixel sizes allow you to cram more detail into a given viewing angle. However, there are limits here. As human eyeballs age, their ability to distinguish very fine detail fades; a 16yo may be perfectly happy with a 1920x1200 monitor and 9pt type, but their 60-something grand-parents most definitely won't be and their parents would have to squint hard. 

When I was hunting for an HD-TV I found a few articles describing how far away from the TV you had to be to tell the difference between 720p and 1080p. What that distance was depended on two things; how old the viewer was, and how wide the screen was. For a 42" TV at 96 inches, only 5 year olds can tell the difference between 720p and 1080p. If they're on the couch that is, and not parked on the floor 3 feet from the TV.

For comparison, a 720p 42" TV at 96" gives an apparent pixel size of 0.017066 degrees. A 1080p panel with the same apparent pixel size would have to be 54.9 inches wide (a 63" panel).

I've read reports of display makers showing off HD+ TV displays with in excess of 2000 vertical pixels. These aren't really commercially available in no small part due to the lack of media available at that resolution, but also the fact that the panels would have to be very large indeed for the average consumer to notice a difference from 1080p in their actual living rooms. Eyeballs are so limiting.

So I am impressed with HD TV, even though I've been a daily user of display tech capable of more detail for years before I got one. Scaling displays up to that size takes work, and getting them affordable to the likes of me takes very high manufacturing quality. The fact they've done it is woot-worthy.

But don't get me started on 3D.