Size

TV screen size is measured diagonally across the face of the screen. It used to be measured in inches but then changed to centimetres viewable (cmv). This should represent the picture size you can see to within half a centimetre. But with the advent of LCD and plasma screens, the clock has been wound back to the 1960s, and the TV industry is using old fashioned inches again.

Here's how the sizes compare. 

Optimal viewing distance

As a general rule, the ideal viewing distance from a normal standard definition TV is around 5 times the diagonal screen size. So for a 68cmv (29 inch) set that's around 3.5 metres. For a widescreen TV it's about 3.5 times the diagonal size.

For high definition TVs the closest you should sit is 1.5 times the diagonal screen size for a 1080i screen, and 2.5 times for a 720p screen. So for a 106cmv (42 inch) screen with 1080i resolution you can sit as close as 1.5 metres without seeing pixels. For the same sized screen in 720p resolution you need to be 2.5 to 3 metres away.

Conduct your own test in the store with rough measurements of where you'll sit and where the TV is likely to be positioned. Move closer and further away from the TV to see if and where the picture loses quality.

Widescreen TVs

These screens have a width-to-height ratio of 16:9 - similar to that used in movie theatres. Movies on DVD are usually in 16:9 and an increasing number of  TV programmes are shown this way.

Although the screens are wider, they are not as high as you might expect. For example, if you want to replace a conventional 68cmv set with a widescreen model in a similar size, the choice isn't easy. A 76cmv widescreen set will show you pictures that are shorter by about 3.5cm than those on your 68cmv set. Smaller 66cmv sets are only just wider than a normal 68cmv set, and very much shorter.

What you really need is a 91cmv widescreen set, but that's not a common size ... Even the next size up, 93cmv (37 inch) is relatively rare and a little expensive.

Common 4:3 sizes and nearest widescreen equivalents:

• 34cmv = 43cmv (14 inch = 17inch)
• 51cmv = 66cmv (21inch = 26 inch)
• 59cmv = 80cmv (25inch = 32 inch)
• 68cmv = 93cmv (29inch = 37inch)