How to calculate Essential Matrix?

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I have used used BRISK features for feature matching as shown in code below and now want to calculate Essential Matrix can you let me know how to do it.I would like the detailed answers.

import cv2
import matplotlib.pyplot as plt
img1 = cv2.imread('D:\IMG_0112.jpg', 0)
img2 = cv2.imread('D:\IMG_0113.jpg',0)
brisk=cv2.BRISK_create();
kp1, des1 = brisk.detectAndCompute(img1,None)
kp2, des2 = brisk.detectAndCompute(img2,None)
bf= cv2.BFMatcher()
matches = bf.knnMatch(des1,des2,k=2)