Confused with various compression levels

There are different levels of compression available in 7zip. I am kinda new to it and don't know the difference. Can someone plz help ?

Thanks, Tookeri. Ur idea is good. But I have files of size not more than 1.5GB - basically movies. Will it show some effect there ?

In any compression utility, the compression level tune how aggresively will the program attempt to process the input data. Higher levels will try to search for more redundancy, more advanced techniques and analyze the files more carefully in an attempt to further reduce the final size, but tend to increase the time needed to process the whole thing.

In short, that option lets you specify a trade-off. Higher compression level or shorter compression time? The "none" option doesn't compress at all, but it's very fast, while the best option will get the smaller file, but taking a long time. It's up to you what to prioritize. I always chose the best option for getting the smaller file.

About your last comment ("basically movies"), it's not a good idea to try to compress any multimedia file, with any program or compression level. Movies, and also images and audio, are internally already compressed (with special-purpose algorithms) that make 7zip-like programs useless. Almost always you'll get a "compresed" file as big as the original, no matter what options you choose.

Alejandro is right. Movie files (and audio files) are already compressed (in most cases). Running them through additional compression is pointless. It might be instructive to look at why this is so ...

Compression algorithms work by finding redundancy in the data and removing it. Subsequent un-compression works by restoring that redundancy.

So when a video is compressed (and the data redundancy is removed), further compression is not possible. There's simply no more redundancy to remove.