Aspect ratio is about how wide to how tall the picture is. There is a conflict about this in video presentations because humans tend to see or notice what is side to side much more than up and down (your eyes are side by side) yet the lenses used to focus an image on a capture device tend to be circular. Another complication is that the capture device tends to be rectangular.
Old movies, up to the fifties or so, and analog TV standards all used an aspect ratio of 4:3. This was nearly square so the lens artifacts were only a big problem near the corners and most of the glass in the lens was put to good use and the film used to record images was also used for images from edge to edge.
With movies, the film could be made wider and each frame less tall and still use all available film surface for images. This lead to wide screen movies. The ‘waste’ was in the lenses used for capturing an image and its projection. Some tricks used for backwards compatibility such as anamorphic techniques.
The 2008 HDTV Buyers Guide, Part 3 discusses how aspect ratios are noticed in modern TV’s. The new high definition TV has a 16:9 aspect ratio screen. This is wider than the old 4:3 but sill narrower than many modern movies.That means that there is a lot of content that requires adjustment to use the entire screen area of a modern wide screen TV,
If you just fit the image to fit the narrowest dimension, you will get bars of blank screen area around the picture. 4:3 images will have bars on each side and wide screen movies will have bars on top and bottom. The image doesn’t fill the whole TV screen.
A second technique is cropping or masking. Crop off the top and bottom of a 4:3 image until its width is the same as your screen or crop off the sides of a wide screen movie until its height is the same as your screen. This method means you loose some of the original picture. The pan and scan technique is cropping with a bit of judgment thrown in so important objects or person near the edge of a wide screen movie can be shown – this helps keep speaking actors on screen, for instance.
A third method is similar to the anamorphic technique, This is to squish the picture to fit. 4:3 images are squished vertically making things look stubby – football players look real hefty! A wide screen movie gets stretched vertically making things look skinny. This can be rather distracting depending upon the content.
Other methods try to use intelligence to stretch, squish, crop, and adjust the image depending upon content and composition to fill the screen without making the adjustments very noticeable unless you are looking for them. This takes a lot of picture analysis and picture processing power. Modern TV’s have this sort of capability.
To show a wide variety of movies and TV shows in their original aspect ratio is one reason for a big screen TV. A modern 40″ 16:9 wide screen TV can show a 4:3 old style TV show the same size as a 36″ old style TV. This is a ‘backwards compatibility’ issue.
Another issue that is less visible is that all TV images are processed to fit. There is a long line of processing techniques used to get the image from camera to your screen and they influence picture quality. It is easy to fall into the trap that digital means the image is a pixel to pixel map directly moved from the camera sensor to the screen but that isn’t how it works and never has been. This is a fascinating field involving codecs, modulation schemes and many other technologies.
Tech support for TV manufacturers indicate that the aspect ratio problem is one the big items in their call list. Customers complain about the black bars or about misshapen objects. Even if you figure out why wide screen movies can have black bars on your wide screen TV you may still be stumped by the set’s menu options for adjusting the picture to fit the screen in various ways.