When working with digitally compressed video in a format such as Flash Video, the higher the frame rate, the larger the file size. To reduce the file size, you must lower either the frame rate or the data rate If you lower the data rate and leave the frame rate unchanged, the image quality is reduced. If you lower the frame rate and leave the data rate unchanged, the video motion may look less smooth than desired. Something to remember here as well is that you can never gain quality when encoding, so if you have a bad video to start with it will not get any more pleasant to look at just because you boost the data rate.

For short;
Frame rate (fps); The less, the more jumpy.
Data rate (bitrate); The less, the more pixels/blur.
Keyframe; The less, smaller file size.

Since Flash Player 9,0,115,0 there is now the possibility to convert HD quality, so i brief version list of the codecs;
Flash 6-7; Sorenson Video
Flash 8-9; On2
Flash 9-10; On2 (new F4V open format since 9,0,115,0)

There is a really good convert guide to get some help on the export settings;
http://www.adobe.com/devnet/flash/apps/flv_bitrate_calculator/

How to work out the best video compression for me?
This is the hard bit, for short I would recommend these steps.
1.) Figure out a compression that you guess would be the perfect one (get help from calculator).
2.) Setup 4 more compressions, 2 that has higher quality than your (1.) setting and 2 that are below.
3.) Line up all the videos and find the video with the lowest quality settings but that you find is still good. Now from this go back to step 2.
4.) After a couple of rounds this way you should have found a compression for your video.

Remember that all videos are different, so there is no magic setting that will work on all your videos. Also when choosing the “best” video you need to keep track on the file size and what the target is going to be.

Frame rate (fps)

Video is a sequence of images that appear on the screen in rapid succession, giving the illusion of motion. The number of frames that appear every second is known as the frame rate, and it is measured in frames per second (fps). The higher the frame rate, the more frames per second are used to display the sequence of images, resulting in smoother motion. The trade-off forhigher quality, however, is that higher frame rates require a larger amount of data to display the video, which uses more bandwidth.

Video looks much better at native frame rates so it’s recommended to keep it if possible. For full-motion NTSC (National Television System Committee in the U.S.), use 29.97 fps.  For PAL (Phase Alternating Line in Europe), use 25 fps.

If you lower the frame rate to lower file size the Flash Video Encoder drops frames at a linear rate to achieve the new fps rate. However, if you need to reduce the frame rate, the best results come from dividing evenly. For example, if your source has a frame rate of 24 fps, then reduce the frame rate to 12 fps, 8 fps, 6 fps, 4 fps, 3 fps, or 2 fps. If the source frame rate is 30 fps, in most cases you can adjust the frame rate to 15 fps, 10 fps, 6 fps, and so on.

If a video clip is longer than 10 minutes, the audio will drift noticeably out of sync if you do not adhere to the 29.97 fps rate or an accurate even division for lower frame rates (such as 14.98 fps, which is half of 29.97).

For short;
NTSC (u.s); 29.97 fps
PAL (Europe); 25 fps
Dropping frames; Reduce by 2 (e.g. 24, 12, 8, 6, 4, 3, 2 fps)

Data rate (bitrate)

The data rate affects the quality of a video clip. When you deliver video using the Internet, you should produce files using lower data rates. Users with fast Internet connections can view the files with little or no delay, but dial-up users must wait for files to download. If you anticipate an audience of dial-up users, you should make short video clips to keep the download times within acceptable limits. There are a couple of presets when encoding but it’s as well possible to set your custom data rate.

When converting you can choose the bitrate encoding from CBR (Constant Bitrate) to VBR (Variable Bitrate). When using CBR it will hold the same bitrate constant over the whole video, the VBR will analyze the video and where there is not a lot of movement it will lower the data rate, on the other hand where it’s a lot of movement it will set a higher data rate. You can also change the Encoding passes from One to Two, when doing so it will first analyze the video before encode, use this as a default for professional videos.

For short;
Data rate; Keep as low as possible without losing your “maximum quality”.
CBR; Constant bitrate, same data rate over whole movie
VBR; Variable bitrate, changes data rate by analyzing movements

Keyframes

Keyframes are complete video frames (or images) that are inserted at consistent intervals in a video clip. The frames between the keyframes contain information on movement and scene changes that occurs between keyframes. For example, if a video depicts a person walking past a doorway, the keyframes contain the complete image of the person and the door in the background, and the interval frames contain information describing the person’s movement as they walk in front of the door.

The keyframe interval value tells the encoder how often to re-evaluate the video image and record a full frame/keyframe. The keyframe interval value can be any number up to 100. When you select Automatic as an interval setting, Flash Video Encoder places a keyframe every two seconds of playback time.

For example, if the video you’re encoding has a frame rate of 30 fps, a keyframe will be inserted every 60 frames. In general, the default keyframe interval value provides a reasonable level of control when seeking within a video clip. If you need to select a custom keyframe placement value, be aware that the smaller the keyframe interval, the larger the file size. If your footage has a lot of scene changes or rapidly moving motion or animation, then the overall image quality may benefit from a lower keyframe interval. In general, a higher keyframe interval produces better image quality because data is not wasted describing the areas of an image that remain unchanged from frame to frame.

An important dependency of the keyframe interval is that it affects the ability of Flash Player to seek (fast-forward or rewind) through an FLV file. Flash Player can only advance from keyframe to keyframe, so if you want to skip to different places and pause the frames, you must use a lower keyframe interval value. If you want to advance to every frame in an FLV file, use a keyframe interval value of 1. When you reduce the keyframe interval value, you must raise the data rate for the Flash video to maintain comparable image quality.

For short;
Keyframe low number; Good for “moving” in video timeline, file size is bigger and need higher data rate.
Keyframe high number; Lower file size, Good “standard” rule is (frame rate * 2), doesn’t work good for something that changes scene or moves fast.

Aspect ratio

The most commonly used aspect ratio is 4:3, 16:9 and 2:1. As a rule you should use the same aspect ratio as the original video, else the video will look distorted. An exception is when converting form DV formats that uses rectangular pixels (non-square pixels).

There are some easy formulas to use if you want to resize the video, first decide your locked side (height or width).

Height is master;

Height with 4:3 as example;

Width is master;

Width with 4:3 as example;

Example; if your video is 720×480 and want to change it to a 4:3 format. We use the height formula and lock 480. This will give us the new video size of 640×480.

For short;
Ratio; Try to keep original ratio, when changing video size, use formulas.

Interlaced / Non-Interlaced

Most broadcast video is interlaced, though emerging high-definition television standards have interlaced and noninterlaced variants. Interlaced video consists of two fields that make up each video frame. Each field contains half the number of horizontal lines in the frame; the upper field (or Field 1) contains all of the odd-numbered lines, and the lower field (or Field 2) contains all of the even-numbered lines. This actually means that a NTSC 30 fps TV is displaying 60 fps.

Noninterlaced video frames are not separated into fields. A progressive-scan monitor displays a noninterlaced video frame by drawing all of the horizontal lines, from top to bottom, in one pass. Because of this, both fields that make up a video frame are shown simultaneously. Thus, a computer monitor displays video at 30 fps, and most video displayed on computer monitors is noninterlaced.

For short;
Interlaced; TV (uses 2 fields)
Non-Interlaced; Computer (uses 1 field)

Audio

The audio settings are often the simplest ones. You can select Mono/Stereo channels, if you don’t need Stereo so select Mono to save some file size. You have as well the bitrate (kbps) for the sound, standard music cd-rom has 128kbps, try to go as low as possible without getting strange noises.