DISCLAIMER: The information that follows does not use the ITU-R BT.601 standards to define aspect ratios because it is that little bit too complicated to explain and frankly aspect ratios are confusing enough as it is. If you actually know what this standard is then you probably already know yourself how to deal with aspect ratios correctly. If you don't know what it is then either don't worry about it or read this page for more info.
Aspect ratios are one of the more confusing things about video. They rarely make as much sense as they probably should. Close investigation into aspect ratios will just get you more confused than you were to start off with, so I'm going to try and keep it simple whilst keeping it accurate in a relativistic sort of way.... I've no idea if this is possible but here goes.
There are two main ways of describing aspect ratio - Pixel Aspect Ratio (PAR) and Display Aspect Ratio (DAR).
Pixel Aspect Ratio
A pixel is a pixel, it is a small block of colour information. However, some playback devices have different shaped pixels than others. Monitors have square pixels and hence everything that is designed for display on a monitor should have a PAR of 1. TVs however have 'rectangular pixels' which have a different PAR depending on the format (NTSC or PAL).
Full screen NTSC dvd footage is usually 720x480 and fullscreen PAL dvd footage is usually 720x576. This footage is supposed be shown on a 4:3 TV... but these resolutions aren't 4:3 mathematically, so what's going on?
The reason for this (I'm simplifying here) is due to the fact that TVs have rectangular pixels and when you show the above resolutions resolutions on a TV they will look 4:3.
In practical terms it means that when you look at dvd resolution images (720x480, 720x576) on a PC monitor in programs like VirtualDubMod they do not have the correct pixel aspect ratio so they look a tiny bit squished/stretched in comparison to the way they look on a TV. This is normal. Some programs will correct this when they are displaying the footage by resizing it so it looks right but if you are viewing it with a "Square Pixels" option then it will look slightly wrong. The good thing is that it doesn't really matter until you display your final product as to what PAR something has. You have a video of a certain resolution to edit with and no matter what PAR you display it in you still have that many pixels.
NTSC footage technically has a PAR of 0.911 and PAL has a PAR of 1.094 but we are going to work from the following principle - on a PC monitor, NTSC dvds need resizing to 640x480 to be 4:3 and PAL DVDs need resizing to 768x576. This is a simplified version of the truth but it suits us well enough.
[You may notice that NTSC is downsized and PAL is upsized - this is purely and simply to keep the vertical resolution the same in case someone uses these values on an interlaced source]
One thing you do need to consider is when you make your own graphics and put them in a video, these graphics will probably be designed for a square pixel display (i.e. a monitor) and when you put them into your 720x480 or 720x576 video they will look all 'squishy' on a TV because it has a different PAR. The easy way to get around this is to make your graphics larger and resize down to have the same 'squishyness' as the footage you are using.
For NTSC you can make graphics at 720x540 and then resize them to 720x480 and they will look just like your dvd footage. For PAL you can make graphics at 768x576 and resize to 720x576 for the same effect. Starting with the larger images and sizing down is the best way to keep quality.
You can, if you really like, resize your dvd footage so that it has a square PAR and then edit but we'll worry about things like that later when it comes to actually preparing the footage.
Display Aspect Ratio
In mpeg video this is often stored as a flag that basically says "this is the aspect ratio that you should display me at". There are two possible Display Aspect Ratios (DARs) on a DVD - 4:3 and 16:9. TV shows will generally be 4:3 and movies will generally be 16:9 unless they are pan&scan or letterbox (see below). Movies with a 16:9 DAR are what we refer to as Anamorphic (or sometimes listed as "optimised for widescreen TV" etc). We will look at those in a moment.
Both PAR and DAR need to be considered when producing video so that it is displayed correctly when it is played.
The Internet (i.e. Monitors) - If you are making a video for people to watch on the Internet and you are using a 4:3 tv series like Trigun then you will need to produce video with a PAR of 1:1 and a DAR of 4:3. Usually this means resizing to resolutions like 640x480 or 512x384 when making your distribution version.
TVs - When making a video for display on a TV you will want to use a full screen resolution with the appropriate PAR (720x480 or 720x576) and a 4:3 DAR for regular TVs or a 16:9 DAR for widescreen TVs (if you actually have 16:9 content). Regular TVs are generally able to convert 16:9 DAR to 4:3 by letterboxing the image (some DVD players can do this too). Essentially what this means for you is that you should probably leave the source the way it is and if you have any original content you are adding make sure it has the same PAR as your source footage.
Conventions - If you are making a video for a convention then it will probably have to be a video with a full screen resolution (720x480 for NTSC or 720x576 for PAL with the usual PAR for this format) with a 4:3 DAR. Conventions usually can't use 16:9 DAR footage so to avoid any 16:9 content being displayed incorrectly you need to letterbox the image to make a 4:3 DAR. I will explain how this works in the Anamorphic source section below.
When it comes to editing, you need be aware of the PAR and DAR when you are mixing footage together. There are all sorts of things ways you can approach this which I will talk about in the sections on preparing your footage for editing.
Some video sources are lucky enough to be encoded as Anamorphic Widescreen. This is a very good thing in terms of picture quality.
So, what is anamorphic?
Let me demonstrate with an NTSC dvd source: Revolutionary Girl
Utena the Movie (aka Adolescence of Utena)
The resolution of an NTSC dvd is 720x480 and is generally
designed for 4:3 footage. However, in order to maximise the amount of
pixels being used, a widescreen movie is sometimes scaled to fit that
resolution like this (images are 1/2 size):
This means that the video is using all of the available quality of the DVD and it also means that widescreen TVs only have to stretch the image horizontally - which is also good for quality.
The display aspect ratio for the image is actually 16:9 so the image, when shown corrected, will look something like this:
That image was made by stretching the image horizontally, just like a widescreen TV would do. If you were watching it on a 4:3 display, you might be able to set it so it adds letterboxing to display the 16:9 image like this:
Sometimes sources will be already letterboxed.
Those have a 4:3 DAR and the footage itself has copious black borders
on the top and bottom just like the image above. This is a waste of
good resolution. However, it is a useful technique when producing
footage for devices (and anime conventions) that cannot display or
compensate for 16:9 DAR.
Another way of dealing with a widescreen source and keeping a 4:3 DAR is to do something called Pan and Scan which will extract a 4:3 image from the source by cropping the edges like this:
You can see the amount that has been removed from
the image by comparing it with the stretched widescreen image above.
The amount of footage lost is even worse when you have big budget
Hollywood movies that actually have an aspect ratio of 2.35:1 (compared
to 16:9 which is only 1.778:1). On DVDs movies with an aspect ratio
like 2.35:1 are usually presented anamorphically with a 16:9 DAR but
still need some letterboxing because they are not 16:9. The principle
exactly is the same as letterboxing 16:9 footage in a 4:3 DAR source
and it all comes out right in the end.
Anamorphic sources may be great for quality but
they become really confusing when you start thinking about both PAR and
DAR. DAR is easy - they are supposed to be displayed at a 16:9
resolution. PAR is more complicated - the usual Pixel Aspect Ratio is
something that will make it look correct when displayed on a 4:3 TV...
but anamorphic footage isn't supposed to be displayed on a 4:3 TV so
what's the PAR for that?
To get our heads around this, it's easiest to think of what a
widescreen TV would do with this anamorphic 720x480 image. What
widescreen TVs do
is resize the image horizontally to 960x480 and which then looks
perfectly 16:9. Of course, 960x480 isn't a 16:9 resolution but 720x480
isn't a 4:3 one either - it's those rectangular pixels again.
The main problem with editing anamorphic footage is the same
as with 4:3 footage - getting graphics to have the same 'squishyness'.
If you are editing NTSC anamorphic 720x480 footage then you
should make graphics at 848x480 and resize to 720x480. If you are
editing PAL anamorphic 720x576 footage at then you should make
graphics at 1024x576 and resize to 720x576.
Of course, there are other ways of dealing with this sort of
source, particularly if you are editing something which mixes 4:3
footage and 16:9, but we will discuss those options later on when it
comes to preparing the footage for editing. For now be content that you
have worked your way through most of the difficult notions of what DVD
footage is all about you are ready to decide how you are going to edit
Alright, so now you should have a general idea about how aspect ratios work, and what sort of aspect ratio your footage has. In the next section, we are going to look at how to actually handle the aspect ratio.