Scripting Reference
Mathf.Log

static function Log (f : float, p : float) : float

Description

Returns the logarithm of a specified number in a specified base.

JavaScript 
    // logarithm of 6 in base 2
    // prints 2.584963
    print(Mathf.Log(6, 2));
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        print(Mathf.Log(6, 2));
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        print(Mathf.Log(6, 2))

static function Log (f : float) : float

Description

Returns the natural (base e) logarithm of a specified number.

JavaScript 
    // natural logarithm of 10
    // prints 4.60517
    print(Mathf.Log(10));
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        print(Mathf.Log(10));
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        print(Mathf.Log(10))