What is -crf used for in ffmpeg

When converting an array of jpegs to a video file (ogg format) I have been using the -crf option. What are the valid ranges for -crf? Currently I am using 23.

I cannot find the bookmark I read to know what this flag does and I have spent some time Googling around.

These are the arguments I am using at the moment:

-f image2 -r 10 -i " + _sourcePath + @"\img%05d.jpg -crf 18  -y -r 10 " + _destPath + "\\" + _filename + ".ogg"

That refers to the Constant Rate Factor (crf).

As others have pointed out (Thanks all), the values will depend on which encoder you're using.

For x264 your valid range is 0-51:

The range of the quantizer scale is 0-51: where 0 is lossless, 23 is default, and 51 is worst possible. A lower value is a higher quality and a subjectively sane range is 18-28. Consider 18 to be visually lossless or nearly so: it should look the same or nearly the same as the input but it isn't technically lossless.

For vpx the range is 4-63 (as @sebastian-hoffner answered):

By default the CRF value can be from 4–63, and 10 is a good starting point. Lower values mean better quality.

In both cases, a lower value means higher quality. In my experience, I see x264 much more often.

Slhck's shameless plug (an SU mod) link looks like it has some good info.

The crf flag ensures a variable bitrate for constant quality, if I understand the page linked below correctly. The default range is from 4 to 63, but it can be tweaked further.

Have a look at https://trac.ffmpeg.org/wiki/Encode/VP9 for more information.