If you want to find out who made changes to a particular file you can use git blame
:
git blame <file> //Substitute <file> for the name of the file you wish to inspect
If you want to see who made changes to a file between a particular range of lines, do the following:
git blame <file> -L 25,50 //Substitute <file> for the name of the file you wish to inspect
The above would show who made changes between lines 25 and 50.