Frame rate is the number of frames (images) that are shown per second when a video is playing. When a video is being played, typically about 20-30 frames are shown per second depending on the need. Within this limit, more the number of frames per second (FPS) less jerky the video is.
Let us consider a video with lot of action, say a soccer game .The video keeps changing at a fast rate. So, the video is typically encoded in 24 fps which is the upper limit of what a human eye can perceive. Any part of such a video will have 24 frames per second.
Now let us consider a video that has mostly e-learning content. The content does not always change at the rate of 24 images per second. For example, lots of slides made in Adobe Captivate are roughly about 3 seconds. If we take out the time for transition effect and a few more animations, a slide might stay without any changes for about a second. If we use only the frames that change we save a lot in file size – in this example we saved about 23 images. More importantly, processing these extra 23 images takes a lot of time for the video encoder (the program that converts these images to an f4v file) – the encoder would do a better job if it had lesser images to process. So when we know that there is nothing that has changed in the slide, we do not add an extra image to encode.
This means we have a video which has a different frame rate within different parts of the same video. For example a slide with just text will have a lesser frame rate, while a slide with a slide video showing a soccer game might have a higher frame rate. We call this variable frame rate. The earlier option of having 24 (or any other number of) frames per second all across the video is what we call fixed frame rate. Variable frame rate files are smaller in size and encoded with better quality than their fixed frame rate counterparts. Almost all video players understand variable frame rate and play them well.
Why did we give a fixed frame rate option if variable frame rate is better in size and quality? – The answer is: some video converters do not understand variable frame rate F4V files. So when you take the F4V file to a video encoder and convert it to a different format, you might find that converted video is not proper. So we gave you an option of fixed frame rate if you want to encode it further or share it on video sharing sites (which convert the videos again internally).
If you don’t want to understand the technical jargon, here you go with this thumb rule – If you are going to convert the F4V output from Adobe Captivate to a different format or if you are going to share it on video sharing services like YouTube, use F4V fixed frame rate. If you are going to directly use the F4V file use the F4V option – it gives you better quality and smaller file size.