A Vidder's Guide: Aspect Ratios Explained

Standard 4:3
Source Aspect Ratio - 4:3 (1.33)
Destination Aspect Ratio - 4:3 (1.33)
With this, the most basic "television" input and output you will see, the source is 4:3 (in this case, 320 pixels wide by 240 pixels high) and the destination (target) is also 4:3 (whether it be 640x480 or 320x240 or any other 4:3 ratio dimensions).

Expanded 4:3 to 16:9
Source Aspect Ratio - 4:3 (1.33)
Destination Aspect Ratio - 16:9 (1.78)
This method takes a 4:3 source and expands it to fill a 16:9 frame. This requires that the width be increased, while retaining the same height, thus causing the appearance of everything being wider than normal.

Sidebar 4:3
Source Aspect Ratio - 4:3 (1.33)
Destination Aspect Ratio - 16:9 (1.78)
This output method will place a properly displayed 4:3 input source onto a 16:9 frame by adding black bars to the left and right until the width and height make a 16:9 aspect ratio. This is how many widescreen television sets will show current 4:3 programming.

Standard 16:9
Source Aspect Ratio - 16:9 (1.78)
Destination Aspect Ratio - 16:9 (1.78)
This is the most common "widescreen" format used by television programs (and all of Farscape season four), 16:9 (in this case, 426 pixels wide by 240 pixels high). It is how it will appear when rendered to a 16:9 destination format (whether it be 852x480 or 426x240 or any other 16:9 ratio dimensions).

Scrunched 16:9 to 4:3
Source Aspect Ratio - 16:9 (1.78)
Destination Aspect Ratio - 4:3 (1.33)
This method takes a 16:9 source and squeezes all of it to fit into a 4:3 frame. This requires that the width be shortened, while retaining the same height, thus causing the appearance of everything being skinnier than normal.

Pan and Scan 16:9 to 4:3
Source Aspect Ratio - 16:9 (1.78)
Destination Aspect Ratio - 4:3 (1.33)
This method will take a 16:9 input source and use a pan and scan (or other cropping) technique available with numerous video editing, capturing, and ripping tools, to produce a 4:3 output that loses some of the image, but retains the natural appearance of the frame.

Letterboxed 16:9
Source Aspect Ratio - 16:9 (1.78)
Destination Aspect Ratio - 4:3 (1.33)
This output method will retain all of the content of the 16:9 frame by adding black bars to the top and bottom until the width and height make a 4:3 aspect ratio. This is how season four of Farscape appeared on standard (4:3) television sets when it aired.