Where to find it:

NodeGraph / Right Mouse Click / Add Nodes / Math / Extension Pack / L /

log returns the natural logarithm of x, i.e. the value y which satisfies  x=e^y

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/log.xhtml

•X

Specify the value of which to take the natural logarithm.

•Output RGB

Separate computation for Input Channels Red, Green, Blue with an Alpha Channel of 1.0

•R

Computation for Input Red channel only, saved into R, G and B channel with an Alpha of 1.0

•G

Computation for Input Green channel only, saved into R, G and B channel with an Alpha of 1.0

•B

Computation for Input Blue channel only, saved into R, G and B channel with an Alpha of 1.0

•A

Computation for Input Alpha channel only, saved into R, G and B channel with an Alpha of 1.0