Chapter 3: Aspect Ratio

Unfortunately we've got some more numbers to look at - but this is the last set we need to understand.

The aspect ratio looks at how wide the display is. 

We all know that most monitors and older TVs used to be fairly square (not quite square, but more square than the next lot). The ratio of width to height for Standard Definition (SDTV) is 4:3. This means that if we have a TV that is 30cm tall, it will need to be 40cm wide. It is also sometimes called 1.33:1 (and if you're good at maths you'll know that means the same thing).

Widescreen TV is now much more common than it used to be, and you can tell if a screen is widescreen because it is, well... wider than the old fashioned type. The ratio here is 16:9 (so if our screen which is 30cm tall will now have to be a whopping 53cm wide). It is also sometimes called 1.78:1.

The last one we need to look at is Cinemascope - this is what they use at the... well, cinema (kind of obvious really...). Cinemascope uses a ratio of 2.35:1 (there isn't a nice 'whole number' version). This means that our 30cm tall TV is now a massive 70.5cm wide. That's seriously wide. Next time you go to the cinema, take a moment to look at the screen. It really is wider than your TV!

Now we understand what the ratios mean (hopefully), some of you might have spotted a problem. What if I want to watch my widescreen (16:9) DVD on an older (4:3) TV? The picture will be too wide to fit! 

Well, we have two choices. We can just chop the two ends off (and hope there is nothing important going on there) or we can shrink the video down to fit, and be left with two black bars at the top and bottom of the video (this is called Letterboxing because it looks kind of like a letterbox.

Of course I could be watching an older (4:3) video on my new widescreen (16:9) telly. Oh no! Now we have two black bars at the sides (this is called Pillarboxing - think of an old fashioned post box). We could always zoom in a bit and chop the top and bottom off. Or we could stretch the video sideways - but that might look a bit daft. 

Most people with a widescreen TV will have noticed this when changing channels - sometimes the TV will try a couple of different aspect ratios before deciding on the best one to use.

Reproduced with kind permission from Mark Clarkson, from the originial source