Wednesday, January 14, 2009

Screen ratios

I was recently asked "So now I've got my widescreen television how come I'm still getting black bars top and bottom on some of my movies?" the answer is "Because they're pains in the fundament"

Okay the normal SD aspect ratio is 4:3 and widescreen is 16:9; for those not mathematically inclined that means for ever 3 units (centimetres, inches, metres, light years) of height there are 4 units of width, or 9 units high and 16 wide for widescreen (no they're not the same as can easily be shown by taken a 4:3 and enlarging it three times to provide a screen 12x9)

If you take a look at the details of any DVDs you'll see the screen ratio - for older TV series this will be 4:3 (or full-screen) some will say 16:9 (widescreen) but others will have 1.85:1 or 2.40:1. to understand what this means compared to 'standard' formats we need to place these on a comparative footing by making them all n:1 so:

4:3 = 1.33:1
16:9 = 1.77:1

So with these formats for every unit of height we need 1.33 or 1.77 units of width in order to display the whole picture. With a widescreen 1080 television we have 1080 pixels (units) of height which means we need 1920 pixels (units) of width in order to display a 16:9 film (1080 multiplied by 1.77 with rounding). With a 1.85:1 movie on the same television if we use the full 1080 height we need 1998 pixels wide, except we only have 1920. A choice has to be made - use the full height and lose 39 pixels from each side, or display the full width and waste 21 pixels top and bottom. For 2.40:1 films this is taken to the extreme where you either lose 336 from each side or waste 140 pixels top and bottom.

This means in percentage terms watching a 2.40:1 film on a widescreen television with full width wastes just over 25% of your screen real estate which is why those bars seem so obvious.

Once everyone's bought a widescreen television and the manufacturers need a new gimmick expect to see 2592x1080 and 1728x720 televisions for sale for full screen display of 2.40:1 films. Actually scrap that the next televisions will be Ultra High Definition with 7680x4320 which is, yes, still a 16:9 aspect ratio.


Orphi said...

Question: Why do they even shoot motion pictures in such strange aspect ratios to start with? Why not just make everything 1:1 aspect? Surely that would be much simpler. What is the supposed benefit of “widescreen” even ment to be?

FlipC said...

From what I understand it's an attempt to mimic the natural range of the human eye. That is if staring straight ahead you mark the extent that you can see you end up with a particular ratio; this got messed up with practical mechanical constraints and what was being used at the time.

At the same time you might as well ask why the UK uses 25 fps and the US uses 30 whereas movies use 24.

Dan H said...

Because the British electricity supply is 50Hz and the US is 60Hz; interlacing makes the framerate half the scan frequency. Before the advent of ultra-cheap quartz crystal oscillators, the only reliable timing signal was the one that comes with your power.

As for film, silent films were shot at different speeds. They were shipped to cinemas with a speed instruction, but the proprietors would show them faster or slower to make the film shorter or longer. The talkies solidified a speed because of the necessity of synchronising the film with the gramophone recording of the soundtrack, and it was set at 24, which happened to be a popular speed at the time and also had technical advantages.

Quite apart from the response of the eye, 1:1 would really box in the action. 20000 Zulu warriors advancing over the African hills would not look quite the same in a square frame. The Godfather talking to all the families would be much less effective if they all had to squash up together to fit in frame. Clint Eastwood wouldn't have the great sense of loneliness riding off into the sunset if you sat there wondering if there was a Little Chef just off the right-hand border.

Life is mostly lived on the flat. The top and bottom of the frame is mostly full of ceiling and floor, or sky and grass. It makes sense to cut it out to make more room for the action. And it means that cinemas don't have to have extraordinarily high ceilings to fit the big screen in.

FlipC said...

Thanks for that Dan, although the first point just pushes the question back to why we use 50Hz and the US 60Hz supplies :-).

Dan H said...

Because American hamsters have shorter back legs and take quicker strides.