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.