# How to choose the right screen size

## Is your telly too small - or too big - for HD?

### The maths bit

That all assumes the 2.7m viewing distance - if substantial numbers of people really are viewing HD sets much closer than that, perhaps the BBC will need to rethink the decision to broadcast its HD programming at 1440 x 1080 rather than a full HD.

Glasses won't help if you're too far away...

Not everyone’s watching BBC HD, and for many of us, an HD TV is also something to use with games, or for watching Blu-ray and other high definition content. And the real question, rather than ‘what resolution do we need to broadcast’ is ‘how far away should I sit from my TV?’

If you’re interested in working out the answer to that question, it’s not that difficult. I’ve made some assumptions here, chiefly that you have good or corrected eyesight, and that your TV set has an aspect ratio of 16:9.

In fact, the important measurement is really the height of the picture, or H. For a 16:9 screen with a diagonal picture of size D, you can calculate H using Pythagoras:

The generally accepted figure for human visual acuity is one minute of arc, so we need to calculate the distance at which one pixel is equivalent to that.

The simplest way to approximate this is with a triangle where the vertical side is the height of a pixel, p, and the other sides are of equal length. Bisect that triangle at the centre point, and we have a right angle triangle, with the angle at the eye equivalent to 30 seconds of arc, and the distance to the eye is d. A bit of basic trig gives us our next formula:

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